I'm a straight female in my mid twenties. I've been dating a wonderful guy for two years—but I recently found something that has put me on edge.

Before we met, he was in a relationship with a terrible, alcoholic, and mentally-unstable woman. They got pregnant early in the relationship and stayed together for about five years. We met a year after they broke up. I feel like I'd come to terms with the ugliness of his past, with his trying to stay in a bad relationship for the sake of his child, and the rest of it. But recently, thanks to the vastness of the Internet, I came across a suggestive photo of my boyfriend with his ex's sister. I asked him about it and he admitted to sleeping with her while he was with his ex. He says it was during a particularly bad period, he was very drunk, she made the first move, etc., but I'm just so grossed out. Cheating is one thing but fucking your girlfriend's sister? And it's not like this was a 19-year-old's mistake. He was near thirty and the father of a child. He also fudged a little about whether it was just one time or a few times. They were a pretty tight family, he spent a lot of birthdays and holidays with them before and after, and he stayed with his ex for another two years after sleeping with her sister. And he never told her about it!

I feel like now I'm questioning his integrity. This is something that I just wouldn't have thought him capable of doing, regardless of how fucked up his relationship was. What do I do?

All Twisted Up

What do you do?

You ask yourself if you believe your boyfriend when he says it was a mistake, ATU, one he deeply regrets, and one he never intends to repeat.

If you simply can't be romantically involved with someone capable of such a terrible/Michael Caine-y thing, ATU, the question is a rhetorical one. You can't stay with him and you'll have to end the relationship. But if could stay with someone capable of doing such a terrible thing so long as it's not a thing they'll ever do again, ATU, and if you believe your boyfriend when he says it was a mistake, one he regrets, and won't happen again, then you stay.

And when your find yourself feeling squicked out by the knowledge that your boyfriend fucked around on a previous girlfriend with her own sister, ATU, you take a deep breath and remind yourself that good and decent people sometimes do shitty, indecent things... and then you pause to consider all the shitty and/or indecent things you've done, ATU, some or most of which your boyfriend presumably remains blissfully unaware.

It's too bad that suggestive photo is rattling around out there in the vastness of the Internet, ATU, but I'm a little curious about how exactly you "came across" it in the first place. If you went looking for dirt—if you were snooping—you found it. Congrats. Now I'm not against snooping in all instances. Sometimes we find out shit we had a both right and an urgent need to know: the BF/GF/NBF*/fiancé/spouse is cheating a way that puts you at risk; they're running up ruinous debts; they're hiding a secret second family; they're sending money to Donald Trump, etc. But just as often we find out shit we didn't need to know—something in the BF/GF/NBF's past, something they regret, something they'll never do again (do you even have a sister?)—and can never un-know.

You learned that your boyfriend is did something pretty fucked up once. Whether you decide to stay or go, ATU, know that you snoop at your own risk—sorry, know that you explore "the vastness of the Internet" at your own risk.

* Nonbinaryfriend.