Within the confines of faith, I think that it is about the lust aspects perceived when the relationship is not useful towards procreation. This is certainly apposed to living a pure life, although I think faiths often miss the areas of grey and only see black or white. Overall, I agree with you, it is up to the individual, if it gets out of hand there are sex clinics for that, religious people should not try to force their beliefs on others, there is enough to worry about trying to please God yourself. Certainly, in the process of attempting to force others to please Him you are not doing very well at it yourself.
I dont think its lust most of the time. I believe that for women they are looking for the softer side of men which is difficult to find because men tend to hide that aspect of themself so they seek it in women. For men its the reverse. However it doesnt work that way, each does have one other half that is opposite sex. I do believe that in the relationships we have that because we have a connection to our opposite that we can see those qualities of our true opposite in those relationships even if its same sex relationships. However when all is said and done your other half will never be same sex.