Devy Metal, why do you assume that sex with a person must become boring after a while? I am asking because it seems that your doubts are not based on facts, what you feel about the girl at this stage of relationship but on assuming what will hapen based on some experiences of other people?
Simply, if long lasting sexual attraction is very important for you, aim to find a person that makes you horny by her very presence. Long lasting love and long lasting sexual attraction can be found in one person, so if sex is that much important for you, just be sure to find a person that is extremely attractive to you, not by "She's a hot chick" factor, but because she makes uncontrolable hormonal activity by her very presence, I don't know if I can easily explain difference. First thing can be mental, or just coming out of natural need for sex, second thing is something bigger, kind of strong chemistry that is subconsious reaction when you are close to someone.
I was in one long relationship that was like that, and even we have gone separate ways, we are still frends, and care about each other, and that also shows that even there was strong sexual factor, we were close in many other ways. So again, just be sure to settle down with a girl/woman that makes you feel out of your mind like that, and that is also a good friend, mate whatever, and you can have your combination: Long lasting relationship-marriage-lot of sex-not feeling boring. Don't let all those stories about married people getting bored, divorsed etc make you think that this is only reality of long relationships, becoming bored, and sexually uninterested in your wife that becomes some kind of asexual roommate to share responsabilities with.