When it comes to relationships, there are certain lines you just don't cross - like flirting with or getting intimate with one of your partner's friends or relatives.
But what happens when your significant other encourages you to do just that?
This is the uncomfortable situation one man recently claimed to have found himself in according to a post shared on Reddit's Relationship Advice forum.
The unnamed man, 23, asked fellow users for some help after his girlfriend of two years suggested he sleep with her identical twin sister.
The sibling had recently been through a bad break-up and her twin wanted to cheer her up and help improve her self-esteem.
Unsurprisingly, the boyfriend was taken aback by the request and at first, he didn't know what to say.
In his post, he wrote: "So I am in this very bizarre situation right now. I've been dating this amazing girl for a little over two years and it's been an absolute blast. She's attractive, cheerful, kind, supportive. We have a great relationship.
"She also happens to have an identical twin and they are pretty close. They've been staying together here since they started university (they go to different universities though).
"Naturally, we all hang out together very often and I am pretty comfortable with both of them. I think of her twin as my own sister in a way.
"Anyway, the twin was dating this guy for almost a year. I never particularly liked him. Well, turns out he was cheating on her with his ex and they eventually broke up (this was six months ago). Needless to say that the girl was devastated.
"She took it very hard. She became kinda depressed and her self-esteem and self-worth weren't doing that great. She started not wanting to go out that much. We tried to do our best to drag her out of course to see and meet new people and why not even hook up with someone. I even tried to set her up with one of my friends, who was single at the time and honestly a great guy overall, but no Bueno."
He continued to say: "My girlfriend always worries about her and tries to help her a little too much, they proceed to fight and then my girlfriend takes it out on me. And we're not even at the good part yet.
"So yesterday my girlfriend was staying at my place. We were lying down talking, you know usual stuff. Until she says she wants to ask me for a big favour. Then proceeds to tell me she'd like me to have sex with her sister.
"At first like any sane man I thought that she was joking... Sadly she wasn't joking at all.
"She probably realized I had a WTF look on my face and then proceeded to rationalize it. She told me that her twin already had a slight crush on me and at the moment I am probably the only guy she truly trusts. She said she thinks it will greatly improve her sister's self-esteem and mood.
"She told me she already talked about it with her twin and the twin was ok with it. She reassured me multiple times she won't get mad about it and she won't even be present, she could just stay at my place while I go to theirs and do the deed! It will only be one time and I could just pretend her sister was her she said.
"She was practically begging me, meanwhile I was standing there listening to her without being able to utter a word, that was how surprised I was."
He added: "In the end I said I'll think about it for a bit and left it at that. I couldn't f***ing sleep that night. That was yesterday. Today I am kinda avoiding my girlfriend, we texted a bit and thank god she hasn't mentioned anything about it... yet.
"I am freaking out.
"Also, I am fearing that if I have sex with the twin, not only things might get really awkward between the three of us, but the twin might later get jealous and start sabotaging our relationship.
"I am getting paranoid at this point, but I just don't want to ruin this nice thing I have with my girlfriend, that's all."
The Reddit post garnered a lot of attention and even ended up being shared on Twitter, where it received more than 1,800 likes.
A number of people replied, with many thinking it would be a bad idea for him to get involved with the twin sister.
One person said: "Yeah definitely do not do this. It'll not only change your relationship with your girlfriend, it'll change her relationship with her sister. Forever."
Another commented: "This is a really unfair and manipulative situation to put anyone in, and there is certainly the argument that there is a very real but different type of pressure on a man in this type of scenario than a woman, as there are toxic elements to our culture that tell a man that he should be thrilled by this kind of thing."
Someone else posted: "No. No. Don't even think about it. Second: it CAN NOT be your job to fix your girlfriend's sister's self-esteem (and it can't be your gf's job either, tbh). It's ok that she's single. Encourage her to use this time to work on herself."
The boyfriend later updated his post, confirming that he wasn't going to do it.
"So I met with my girlfriend earlier today and it went pretty good I think," he wrote. "Turns out she was feeling like s**t the whole time yesterday, deeply ashamed and terrified. She started crying saying she was regretting it all day and she now is 100% sure she doesn't want to share me with anyone and was terrified that if I said yes then she wouldn't be able to do anything to stop it since she suggested it.
"I reassured her that I love her, I think she is perfectly normal and I am not intending to leave her or anything like that. Once things calmed down a bit, we talked about what prompted it. Apparently, she and her sister talk an awful lot about sex and relationships and are very open about it, which in turn means they are talking about our relationship and me a lot.
"We are both meeting the sister in the evening to clear things up, hopefully, things go back to the way they were, I mean it's not like anything bad has actually happened."