I just realised I hadn't posted my answer to that in this thread, and the other threads where I have posted it are gone. This is going to be a pain to type out.
The sexual act is not designed for casual sex. There are a number of biological factors that indicate this.
I'll preface my point by point comments by saying that pretty much none of the following are true, but regardless it doesn't matter. Biological design has no impact on what humans ought to do.
Even so, it is clear evolutionarily that humans, particularly males, are designed for "casual sex". Optimal male reproductive strategy is to impregnate as many females as possible.
Regardless, sexual acts are certainly designed to be pleasurable to humans.
-The fact you need artificial agents to prevent procreation (although it doesn't always work, 65% of all abortions come from contraception sex, which is like over 30 million).
For one this is incorrect. Ever heard of coitus interruptus (aka "pulling out")? Studies have it as like 96% effective.
Anyway, it doesn't matter whether things are "artificial" or not ... and even if it did, humans are part of nature anyway, so the things they invent can be considered natural.
-The male cannot choose to ejaculate or not. Once he reaches a specific point of sexual pleasure, he automatically ejaculates.
This simply isn't the case ... there clearly is some ability to control this, as evidenced by 50 minute porn videos.
-He cannot choose to fire blanks.
Why would this matter?
-Once he has ejaculated, he is rid of his sexual desire.
WTF? I mean, surely it is lessened, but "rid of" is a very strong wording. Sometimes people will have sex 3+ times consecutively.
-The most intense pleasure is the ejaculation, suggesting this is the end goal.
Debatable.
-Prolonged sexual stimulation without ejaculation in the male causes pain in the genitals and increases the chance of prostate cancer.
Ok.
-A female is most "keen" during her most fertile stage of the cycle.
Sure.
Nothing about these factors suggests that pleasure is an alternate end of the act. If it was, we wouldn't need an artificial agent to prevent procreation, we would be able to choose when to ejaculate, and to fire blanks. Pleasure would continue post-ejaculation, the ejaculation would not equate to the most intense pleasure, and there would be no penalty for prolonged stimulation without ejaculation.
It's pretty clear that "pleasure" is at least one of the results...
I want an explanation for these factors. Everyone I debate on this issue seems to avoid providing an explanation for these factors.
It doesn't matter anyway. As I said before, appealing to some notion of design is pointless. My fingers weren't designed to type on this keyboard, but that doesn't mean that it is somehow WRONG to do so.