In the experience of someone I know (

) most couples prefer to meet up first in a non sexual environment. They all suss each other out, discuss expectations, go home and talk about compatibility and whether they are keen and then if they are another meeting is arranged. Sometimes you will have the male partner finding the other female really attractive (obviously this is important as it is after all just physical) but the feelings are not the same between his partner and the other chap. Obviously nothing will happen then and you will politely tell them that it is not going to work out. Most people (from what the friend told me

) are very mature about this sort of thing.
Obviously on the odd occasion it may be more spontaneous, like a pick up at a pub etc.... but those are usually less successful as those types do not understand the "rules"