We do not discuss it because its placed in a moral region. Not so much what we do but what we don't do. And what we do must be evaluated morally not just left to itself.
The fact is, earlier civilisations did not necessarily have these problems or not to the extent we do. Had they desired to, many ancient cultures could have provided brilliant works on the sex act. Egyptian creator gods brought the world into existence through masturbation for example. Gilgamesh epic can be seen as a treatise on homosexuality as a means of escaping the power plays of heterosexual sex, creating thereby close bonds that went beyond use or friendship. Greek expositions on Troy compare heterosexual values with homosexual values without choosing and sticking to either means of copulating.
Personally, I think that sexual differences between the sexes and differences between people of the same sex cause much of the taboo. It is difficult to understand sexual phenomena and sexual pleasure that we do not ourselves experience. Equally, the nature of intercourse as an act of male domination and female submission may be see erotically by the penetrating male but is not always seen positively by the receiver (male or female).