There's a remarkably long-standing notion in Western society that says -- get this! -- sexual relations between a husband and wife should be exclusive. This notion tends to carry over to boyfriend/girlfriend and SO relationships, even though they're really nothing like a marriage (in the sense that there's no spoken commitment or vow of permanence involved).

So why isn't this always the case? Some people are voyeurs and enjoy watching other people having sex in front of them. Some people like imagining they're that other guy or other girl and watching "themselves" having sex from a third person perspective. Some people just like getting to live out their MMF and MFF fantasies.

To these or any other similar ends, some people enjoy sharing their SO with another man or woman (or both). If you don't, make sure you're clear on that in the beginning. It may help make sure you're not surprised by your SO suggesting it later on after the novelty of sex with you has started to wear thin.