Re: Is it human nature to be with the same person for so long?
Judging from what you say, your husband doesn't seem to be a very giving, sensitive person. Some people just aren't out there for anyone but themselves, just doing what it takes to get what they want and need.
I'm sorry to say it, but if he's addicted to p 0 r n (I don't believe he's stopped, but is getting it elsewhere...he probably knows you can check up on him on the computer; check the fertilizer bag!) and propositioning YOUR SISTER(!), he would do it with anybody and probably has already. His sex drive comes before all else, including you and the children.
He gives all signs of being totally self-absorbed (farming and sex are his only interests) and isn't likely to change for anyone. I don't really think people like this are capable of love; his actions prove it.
If you can live with this for the rest of your life, fine. But don't expect any more from him. Relationships don't get better without help and he's not going to participate in family outings, ever, unless there is someone there he can proposition!
It just isn't going to get better on it's own... and it's typical that people are more "romantic" when wooing their catch. That's why we are cautioned from jumping into relationhips/marriages until the honeymoon/seduction period is over and we see them as they REALLY are.
Good luck.