Dangerous liaisons often have consequences, so you have to know how to deal with them.
They're not available
The decent thing to do would be to back off, even if you want to be with them. If they want to leave their partner then let them end the relationship without dragging you into it. If you still want each other when the dust has settled, then you can act on it.
If they think it was all a mistake and want to remain part of the couple, or if you completely regret it, you'll both have to talk through what you are going to do. You have to trust your instincts here and decide whether being honest with their partner is worth the hurt it will cause. Would you only be telling their partner to make yourself feel better? Or do you feel they deserve the truth?
If you decide to make a go of the relationship, it could be difficult for you. In the eyes of many of their friends you'll be the bitch or the bastard who broke up a long-term relationship, which could make it harder for you to be accepted into their group. Then again the friends in question could be celebrating their mate's escape from a bad situation, making you the hero of the hour. Just be careful and make sure you're not a rebound fling.
You're not available
OK, time to think about why you did this. Were you bored? Scared of commitment? Looking for affection you weren't getting elsewhere? Wanting to break up with your current partner? Or are you genuinely in love with someone else?
If you know you don't want to be with your partner anymore then break up with them. Don't arse around behind their back. They deserve some respect.
If you have fallen for the person you pulled, ask yourself if this is a passing crush or the real deal. If you still love your partner and want to stay with them, then let it go. But if it's over, break up with them and take some breathing space. Get your head together and see if you still want the other person when you're feeling more normal.
If your relationship is making you bored or unhappy you need to speak with your partner and address the problems in your relationship. Going through a rough patch doesn't give you an automatic right to sleep around and the damage these actions may cause could turn that bad patch into a terminated relationship. We have tips on successful monogamy here.
You're the best of mates
There are a few possible outcomes here. You could both realise you were meant to be together, which is great for you both.
Some people, rare though it is, can work the fact they slept together into the fabric of their friendship, make jokes about it and remain easy in one another's company. It's also possible the ensuing awkwardness and embarrassment will make being friends difficult for a while, but if you're both prepared to work through it together then there's no reason for you to drift apart.
And prepare for problems if you thought it was a drunken mistake and they mistook it for the start of a beautiful relationship. Or the other way around.
It's your boss
Screwing the crew is generally not advisable, even if it might secure you a big fat bonus. Generally sleeping with your boss is an entry into a whole world of pain, confusion, blurred boundaries and being gossiped about. Why would you want to pack in a job you love for the sake of a fumble in the stationery cupboard?
Some people find authority and ability are aphrodisiacs, but try not to think that way. Focus on the rubbish things like how often they say "erm" in meetings, or the fact they haven't washed out their coffee cup once in all the time you've been there. Just try your hardest to keep everything on a professional level.
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