A friend of mine — a guy who used to occasionally step out on his woman and hook up with other girls — had the strangest theory about the female species.

"Women don't cheat," he told me, when I asked if he was ever worried she was doing the same thing to him. "It's just not in their nature."

I laughed. Of course, I wasn't surprised when he found out she'd been two-timing him for most of their relationship, after their inevitable breakup, but he was completely shocked.

There are many things that men don't know about women — mainly because we don't want them to know — so there are secrets women keep well hidden.

But during my two-and-a-half years of interviewing countless women, I discovered that there are many, many more things that we ladies keep secret from men.

Why do women keep secrets from men?

Most of the time, women keep secrets that are personal to them: how they see their body, what they do in their free time, or any guilty pleasures they may have.

It's okay for women to have dirty secrets stashed away where men can't find them because there are just some things you want to keep to yourself — and that's perfectly understandable. These secrets might be embarrassing or too personal, and only when you're ready should you share.

Either way, women will always keep secrets from their partner, but most of the time these secrets aren't anything to worry about; rather, they are harmless thoughts women have about themselves and don't think of sharing with others.

It might be something like wanting their partner to be kinkier in bed, or that they want to go out with their girlfriends and get a little crazy. Sometimes, a woman's closest friends know more than what their husbands or boyfriends see. 

RELATED: The Secret 25% Of Women Would Keep From Their Loved Ones

Here are 20 dirty little secrets women go out of their way to keep from the men in their lives.

1. We masturbate, whether you're at home or not.

Photo: Forewer / Shutterstock

As soon as we're alone in the house — husband leaves for the office, kids go to school, roommate goes out of town — and we have quality free time knowing no one is going to walk in on us, we touch ourselves.

Sometimes we even just do it if you're still in the house if that quality free time is never going to come. Usually, it's while we're in the bathroom.

2. A good majority of us prefer to pee outside.

And in the shower. And sometimes we really just want to do it in the hot tub, but we try to not do that out of respect for everyone else in there.

3. We pluck stray hairs from everywhere on our bodies.

From our toes, our chin, our lips, moles on the backs of our legs, and our nipples. And we really, really enjoy plucking a stubborn ingrown hair. Getting that sucker out is, for some gross reason, such sweet satisfaction.

4. When we're in love, we smell your clothes.

Or the pillow you were sleeping on when you're not around. If you were to catch us doing this, we'd be mortified.

5. We've wished that we could be more open and casual about hooking up.

Photo: NDAB Creativity / Shutterstock

Maybe go to a swingers party, have a threesome, or be a dirty stripper for a night, but with no emotional consequences.

RELATED: The #1 Secret Every Woman Should Use To Keep A Man Interested

6. We hope that your guy friends secretly want to sleep with us.

And very often, we will dress for them and subtly flirt just so they will. We don't want to actually sleep with them — we just want them to want us.

7. We aren't always insulted by those catcalls from construction workers.

As long as they aren't rude or nasty, it can be kind of flattering. We also like it when you get a little jealous, to a degree. Not in an irrational or psycho way, just a bit to show you're protective and you care.

8. When you're not around, we fart.

The longer, the louder and the stinkier, the more enjoyable.

9. When we have girls' nights, we do bad things that you wouldn't approve of.

Photo: Getty

This includes spilling all of your embarrassing secrets, sneaking cigarettes or other substances, and drinking way more than we let on. Grinding with strangers at a club can also sometimes occur.

10. If you're hot or the hookup is good, you can be a total idiot and we will probably still date you.

But we'll never marry you. Brains and kindness will always trump hotness when it comes to marriage material.

11. We like to sleep together on the first date to determine whether the chemistry is there.

Or sometimes we're just plain excited. And we hate being judged for it.

12. We hate waxing our privates.

Hate it, hate it, hate it! But we like that when it's cleaned up, you go down on us more readily. In a perfect world, you would go down on us with regularity on naturally poofy pubes.

13. When a guy says he doesn't want kids, it typically makes him less attractive.

And it's usually a deal-breaker for women who are of child-bearing age.

14. The majority of us don't really care about how much money you have.

Photo: Africa Studio / Shutterstock

As long as you're kind and emotionally generous and work hard. Laziness and lack of motivation are inherently un-sexy.

RELATED: 11 Mind Games Men Think They Play (That Women Simply Choose To Ignore)

15. A lot of us are fakers when it comes to our love of sports and being outdoorsy.

We'll certainly give it the old college try if we want you bad enough, though.

16. We actually like your flaws.

A little belly, gray hairs, even a receding hairline. It reminds us that we all have body issues and that we shouldn't be so insecure or hard on ourselves. Being human is cool, but being whiny about your paunch or constantly fussing over your gray temples is as annoying as us always asking, "Does this make me look fat?"

17. During intimacy, we might be thinking about something other than you.

A gross and pervy situation, another man, being dominated... who knows. But most of us love to fantasize. It doesn't mean we're not sexually attracted to you, we just need the mental images to take it up a notch.

18. We regularly check in on what our exes are up to via social media.

Photo: sergey causelove / Shutterstock

As long as we have the technology, they will never be fully out of our lives or minds. This doesn't mean we still love them; we're just curious.

19. We don't consider drunk kissing cheating...

...as long as we're the ones doing the drunk kissing. We consider sleeping with another man cheating.

20. We cherish our independence and 'me' time more than you'll ever know.

We say we miss you but are often secretly glad you're gone so we can totally relax and be ourselves. But we still love it when you come back.

RELATED: The 15 Biggest Secrets Men Keep From The Women They Love (And Why)

Emily Hingle is a writer, editor, and social media coordinator at LCIA.

This article was originally published at The Frisky. Reprinted with permission from the author.