If can look and touch a mahram then why can't I enjoy sexual realations as well? please provide specific evidence to support your answer.
I am not sure if I got your question correctly, but I believe you will find your answer (a reference for tahreem) in Surat Al-Nisaa (Chapter 4) Verse (Aaya) #4, and I quote it here:
" Prohibited to you [for marriage] are your mothers, your daughters, your sisters, your father's sisters, your mother's sisters, your brother's daughters, your sister's daughters, your [milk] mothers who nursed you, your sisters through nursing, your wives' mothers, and your step-daughters under your guardianship [born] of your wives unto whom you have gone in. But if you have not gone in unto them, there is no sin upon you. And [also prohibited are] the wives of your sons who are from your [own] loins, and that you take [in marriage] two sisters simultaneously, except for what has already occurred. Indeed, Allah is ever Forgiving and Merciful. "
Mahram is derived from haram, it literally means "forbidden people", i.e. people with whom sex and marriage is forbidden.
Quran 23:5-6 And they who guard their private parts. Except from their wives or those their right hands possess, for indeed, they will not be blamed.
Also according to the Quran, marriage with mahrams is forbidden:
Quran 4:23 Prohibited to you [for marriage] are your mothers, your daughters, your sisters, your father's sisters, your mother's sisters, your brother's daughters, your sister's daughters, your [milk] mothers who nursed you, your sisters through nursing, your wives' mothers, and your step-daughters under your guardianship [born] of your wives unto whom you have gone in. But if you have not gone in unto them, there is no sin upon you. And [also prohibited are] the wives of your sons who are from your [own] loins, and that you take [in marriage] two sisters simultaneously, except for what has already occurred. Indeed, Allah is ever Forgiving and Merciful.
Since a Mahram is a close relative and a close part of one's household, it is permitted to look at them and to touch them when necessary (on the face, arms, etc.) Looking at and touching the private parts is not permitted, and touching anywhere with desire is also not permitted.
Section on Looking and touching mahrams in the Hidayah:
A man may look at the person of his kinswoman. IT is lawful for a man to look at his female relation either in the face, head, breast, shoulder, or legs : for as it is usual with relations to visit one another without any previous intimation, and unattended with any retinue, and as women, in their house, generally wear a dress adapted to service, if, therefore, the sight of these parts were culpable, it would impose too great restraint upon them. It is different with respect to other parts ; and hence proceeds the illegality of looking at the back or belly. (It Is proper to observe that by the term relation [Mohrim], as here used, it to be under stood any person between whom and the beholder marriage is utterly and perpetually illegal, in consequence of affinity by either blood or marriage )
Male and female relations may touch each other (if there be no apprehension of passion.) EVERY part in a relation which it is lawful to look at may likewise be touched ; unless, however, there be a dread of its inflaming the passion of either, in which case neither the sight nor the touch is approved.
Additional reference: Awrah in front of Mahrams islamqa.
Others have given enough of references which are very good. I'll try to give a rational answer.
Same can be said for a 4 year old girl. You can touch her, so why not have non-penetrative sex with her in her sleep? There is a natural shame in doing it (There may also be a desire, but still the shame is there). Same shame exists for your mahrams. The only difference with mahrams is it's consensual and with an adult.
- It would make inheritance complicated. A sister may also be a mother, etc.
- Families would be in contact with a smaller population and learn less from others. It's like them living in their own village. (Normally people/groups who only marry their own kind/group have a lower level of tolerance for othering. The more diverse you more you're adaptable you become and more you can understand others. (I don't mean accept everyone, but some differences make total sense, some make zero sense) If you move to a new city you learn something new, if you learn a new language you learn something new, if you marry a different person you learn something new. But with with incest you're planting a seed of social restriction onto yourself and your children.)
- It's not the way Allah wants. I mean you can eat a sheep that has been slaughtered halal. For it to be slaughtered halal you must say bismillah before. If you don't then the meat is haram. For a marriage you must say certain words before. If not it's haram, it's zina. And it's not like these certain words are just for the sake of a checkbox, no when you say these things there spiritual effects taking place. As a Muslim we should believe in the unseen. Marrying your own mahram has certain unseen affects.
- There are genetic concerns. Inbreeding which is very similar creates many genetic issues. See here and here Now imagine if this happens 2-3 generations in. The effect would be magnified. From the article it says:
However it is estimated that 55 per cent of British Pakistanis are married to first cousins, and a third of children with recessive genetic disorders are born to that community.
So to conclude it's to be avoided for complications in inheritance, creating social restrictions, it won't have Allah's blessing, genetic issues.