I don't agree with this at all given the context of the OP's question. It makes no difference if one or both of the parties (friends) are involved with other people. If they are friends, there will most likely be some sort of sexual tension. No matter what their status is outside of that friendship. Whether they choose to ACT on it MAY depend on what their status is...but not whether or not the tension is there.
Yes, the tension would remain, but the chances of actually becoming sexually active with one another decrease if either or both are involved with someone else.
"A casual stroll through the lunatic asylum shows that faith does not prove anything." - Friedrich Nietzsche