Because most people do not understand what love is at all. You don't understand either (I'm talking about the author of the question). Love is not about sex or even physical contact. Love is about accepting a person as he is.
And the fact that someone is there fucking without love, well, let them fuck, what does it really matter to you? Nobody forbids you either, as long as you and your partner are satisfied with everything, then there are no claims to you. In public places, just do not do this, and regardless of whether you are heterosexual or same-sex, it usually interferes with people a little.
By the fact that people do not understand at all what love is? And you in particular, because there is no love, neither heterosexual nor homosexual. What you mean by this is called sex drive. And whoever wants to satisfy his physiological needs, and love has nothing to do with it.
Many people understand. Such people do not care who, with whom and how, and if they know, they will support. Well, those who condemn ... Basically this is the result of projection (roughly speaking, the judgments of others by themselves). In addition to it, extremely high suggestibility and ignorance, blind religiosity and political fanaticism are possible (from more to less often). But in most cases it is still a projection.
Many people understand. And they repeat about it at every corner. Can't you hear? Sex does not have to be aimed only at conceiving children, otherwise it would be done in nature only once or twice in a lifetime, and a person as a natural species would not have sexual activity all year round. For animals, it's only spring.