I caught part of this movie on TV years ago and I've never seen it since, or been able to find out what it was called. It looked like it was made during the 70's, but I can't be sure of that.
Most of humanity has been wiped out by a contagion. It is suggested (but not confirmed) that it may have been a bio weapon. Men and women have split into separate tribes, and will kill each other on sight, as well as any of their own who come into contact with a member of the opposite sex, as they each believe that the other side are carrying the disease.
In actual fact, everyone is already infected, and the virus is activated during sex, horribly killing both participants shortly afterwards.
I remember two scenes specifically. In the first, a man attacks a woman he finds on her own, but is discovered by his own people, who kill him. The woman escapes and makes it back to her tribe, where they kill her rather than let the disease run its course.
In the second, there is a small farming community that contains both men and women, who, we are told, are aware that they can live together safely as long as they are celibate. They are attacked by the male tribe seeking supplies. When some of them react with surprise after the raid that there are both men and women among the dead, their leader refuses to acknowledge this, saying something like "I can't see any men".