Evolutionarily speaking, a woman who get penetrated by an unfit man, is more likely to produce an unfit baby. Whereas a man can penetrate women indiscriminately fit or unfit, since he's not the one getting stuck with those babies. 

So a man is inherently opportunistic when it comes to sex, whereas a woman is inherently discriminating. And culture only tends to reflect these tendencies, not generate them. And saying otherwise is akin of blaming western culture for not encouraging men breastfeed babies. Culture stems from biology, not vice verse. 

Now, speaking about gay sex, Viagra probably makes the job staying erect on demand much easier. And I am not saying gay sex is not natural, since it happens all the time in nature. It's just that there is no evolutionary pressure for male penis and male (or female) anus to conform to each other, like there was strong evolutionary pressure for male penis and female vagina to conform to each other. 

Since men with too big or too small penises in relationship to most vaginas did not tend to leave many offspring. And so vagina who was too small to accept most penises or too big to keep one in place, did not had much success either. On the other hand the fact that most male penises are too big for most anuses (and hence cause pain without lubrication), did not result in any fewer babies. In fact when it comes to female anus, there might had been evolutionary pressure for the female anus to be too small for male penis (and vice versa) in order to prevent misplacing the sperm. Since men hardwired for plugging the holes would have had challenge penetrating anus if their partner experienced pain.