Mr f1r3br4nd compared having sex to going to the theatre to see a movie. Sometimes you go alone, sometimes you go with a friend. Sometimes your friends go without you. It would be ridiculous to insist that you and a friend only see movies with each other.
The problem with this analogy is that it assumes that sex, like going to the theatre, is a casual, non-personal activity.
Let me illustrate. If someone were to, say, drug you and take you to a movie with it (it being a gender-neutral pronoun), would you care? You'd most likely be a bit annoyed, perturbed, disturbed; perhaps you'd say something like "Jeez! You could have just asked!". Definitely not suffering from any massive emotional scars, right?
(Proofreading thanks to
Ichiro2k3.)