You can think what ever you want, it feels good because it stimulates nerve endings in the anus an penis, Period.
It feels good because the nerve stimulation is interpreted by the brain as pleasurable (for most people).
For some (like me), nerve stimulation is experienced as uncomfortable bordering on painful at times whilst at others it feels okay (sometimes even good) and eventually leads to ejaculation. However I don't think I experience an orgasm unless the equivalent of a sneeze is considered an orgasm.
As I understand it, the pleasure is experienced in the brain due to chemical release (which is perhaps absent in me). The point being that it is the brain that is reacting to stimuli, and being so complex, it is possible for the reaction to be different between individuals due to changes in the brain from construction differences or environmental influences.
I understand that many women can't orgasm until they learn how to stimulate themselves in the right way: the mechanism exists but without the environmental influence of learning, it is not realised.
Some with spinal injuries can not experience orgasm through penile stimulation, but have learned that stroking another body part can also be interpreted to trigger an orgasm.
More recently, many men are discovering the pleasures of prostate stimulation, with some finding that after training their brains to recognise the nerve impulses as pleasurable, they can trigger an orgasm without physical stimulation simply by imagining it.
Thus it is not simply a matter of nerve stimulation, period, but of the brain interpreting those signals appropriately. Whilst there is obviously a fundamental physical mechanism of specific nerve stimulation leading to pleasure, the path to pleasure is far more flexible and ultimately centred in the brain and perception.
Consequently I believe it is theoretically possible to develop a pleasure response to a wide range of stimuli, not only physical; however it may need the fundamental mechanism to be activated first so that the brain knows what pleasure is, before it can be reproduced through other means. It is therefore conceivable that some people may have pleasure responses to different stimuli from the norm and these could be learned through environmental influences.
I suspect societal influences have as much effect on behaviour as initial biological programming and the brain is somewhat plastic to reprogramming. Given the right circumstances, I think most men would be able to derive pleasure from sex with other men, however society has condemned this for so long that there is a mental barrier to exploring it and thus the usual inbuilt opposite sex attraction predominates. When incarcerated though, some men overcome the mental barrier when desire and opportunity exceed social mores.
What I am trying to say is that I think we are biologically programmed at birth with some behaviours that are sex specific; that sometimes these behaviours become switched between the sexes for reasons unknown; but ultimately the brain is somewhat plastic and can be reprogrammed to modify or override those pre-programmed behaviours to varying degrees.
I'm not convinced we are born a top, bottom or versatile but it is possible receptive or penetrative are biological behaviours linked to sex, which can become confused between the sexes, but these are ultimately influenced by learning and environment.
I remember reading about experiments with cattle in which young females were injected with testosterone, which changed their behaviour to include mounting (which is normally associated with males). So there we have an example of how basic biological programming can be altered.
When I first accepted my sexual orientation, I had no interest in arses and I couldn't understand other gay men's attraction to that part of the anatomy. Over time I have begun to perceive that situation differently as I have been exposed to stimulation. I'm still not completely at ease or attracted to them, but I acknowledge their role in providing pleasure to others and am not averse to some engagement for that purpose. Presumably with increased engagement I might even grow to like arses through association with my own pleasure. Anecdotal evidence of heterosexual men who like anal sex with their wives as a treat, suggests anal sex may have some benefits compared to vaginal sex. I have yet to experience this for myself, which could change my perception of arses.
When mankind mapped the genome, they thought they would understand everything there was to life, but they discovered epigenetic factors that switched genes on or off. I doubt that is the end of the matter and that there is still more we do not know and we do not know what we do not know. Similarly I think biology and behaviour is more complicated than nature or nurture, but a combination of not only both but diversity of nature beyond a simple binary result.
