Originally Posted by Shey

Ok, speaking from experience here:

If you are the one pushing for the threesome it will most likely kill the relationship unless you do it right. By doing it right, I mean you have to make sure that during the whole time 70% of your attention is focused on her (your partner). Some guys are so excited to be wetting their stick on someone else they forget their manners sort of speak.

If she's pushing for it, the above still applies. The most important part is that before you start, set the ground rules and ask her how she sees the threesome going and what she wants to get out of it. The first couple of times it happens (if you are lucky) it's going to be fun but a little tense, after that, if you both like it, it should be smooth.

Also, after it happens, make sure to talk to her and get feedback, make sure she feels you care about her more than the monster between your legs.

Of course all of this applies only if you really care about her, if not just do it and hope for the best...