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I tend to get into a new relationship like 2 weeks after a breakup, and I do not feel guilty. That may be one of my flaws

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See, by classifying your inability to feel guilty as a "flaw", you may betray a subconscious guilt about it. 

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Yet women get really horny during certain times while they are pregnant

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Well it makes a lot of evolutionary sense to be horny while pregnant. Basically, if you're pregnant you can have sex without consequences of having a baby while perhaps scoring some extra resources from men you have sex with. 

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Most women do not have two babies a year. I think the dynamics are equal between men and women when women are not producing yet

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It's about the potential. Had we not have modern birth controls and access to abortion, most women may have had a baby every year or two. 

Men's desire to have sex is not diminished by the knowledge it won't result into babies, as his desire for fat & sweet food is not diminished by knowing he won't have to face hunger.

Again the guilt is biologically ingrained. For example, why would anybody feel guilty if they had sex with their mother or sister ? If two adults consent to have sex with each other so what if they are related ? Yet incest is taboo, because inbreeding is so dangerous. And using birth control during incest, makes little difference in ameliorating the guilt. So with issue of "sluttiness". the fact that you are single now, yet had sex some months ago, may make you feel guilty to start a relationship again. Why? Because in nature women can get impregnated months prior, and when someone else comes in they may start spending resources on a baby that doesn't share their DNA. So biologically speaking, the woman cheats that man by burdening him with a baby that it's not his own. Hence the guilty feelings, and the condemnations from other. And since they didn't exist in our evolutionary past, birth controls are irrelevant when it comes to the arising emotions. There is an evolutionary truism that is's worth remembering: "Sperm is cheap, Eggs are expensive". That is a man can father, at least theoretically, thousands of babies. While a woman is restricted to having a baby or two once a year, plus subsequent care. Based on this, there is an inherently unequal dynamics between men and women. Of course, because it's natural doesn't mean it's desirable. It's just we have to know what instincts drives us, to better understand the source of our happiness and unhappiness. And perhaps confronting the sources of our guilt, may break at least partially their spell over us.

Well I was stating in statistical terms. Most men may not want to have intercourse if proposed by an old, obese woman with poor hygiene. However most (if not virtually all) women may not agree on the spot to have intercourse even if proposed by a male supermodel. 

In fact they had a study when a cute girl on campus offered guys to have intercourse, and the vast majority agreed. Whereas when a cute guy asked to have sex to girls on campus, not a single one took up on his offer. And I doubt that after the sexual liberation, women refused (at least primarily) because of some social standards, rather than their own, ingrained standards. 

I do agree that religion may influence a lot of our mores, but as they saying goes: "man created god in his image", that is the vengeful, sexist, homophobic and characterize by all the other attribute of the typical ancient patriarch. 

Regarding animals they may deploy different strategies, for example females cats allow to be screwed by a bunch of toms, and then they release their eggs, so the fittest sperm gets the prize. 

Bonobos stereotypically make make much more love than war and chimps vice versa. Humans are equally related to both chimps and bonobos. So they can be a mixed bag. Still it's hard to deny the typical emotions humans experience in the interactions with the opposed sex. Free-love communes have been tried and failed. And situations when one husband has multiple wives are much more common then vice verse. So, while random stuff does happen, the evolutionary logic remains strong & sound amidst all the noise. 

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.