If I had to sum it up in one sentence, it would be that "marriage" is not all about love, sex, and sharing a living space (and a NOT AT ALL about having children, at least to a lot of people - those who are married and do not have them and those who have them but are not married - but that is a topic for another forum I belong to).
(Longer than one sentence):
MY marriage really IS about the benefits that society allows to two people who share the contract on that piece of paper. My husband and I did not do that because of our "love" for one another. While we do HAVE this love, and sex, and sharing of a living space - the real reason we got married is so that he can speak up for me if something happens, although we have discovered other "perks" along the way such as insurance benefits, etc.
I don't know if this is the type of answer you wanted for your post, but for me it explains how I can still want to be married to my DH yet be able to have feelings for someone else that are normally reserved only for one SO at a time. The corollary to this is that just because I can have a personality chemistry and sexual attraction to ANOTHER person, doesn't mean that that person is suited to look out for my back on a daily, mundane basis. Nor is it necessary or appropriate to go and merge every aspect of two lives (financially, logistically, etc.) just to have a satisfying healthy relationship.
There are certain things I need (that are emotionally related to sex but not exclusively sexual) that my husband can't or won't give me, and it would not be fair to him to demand that he be someone he is not just to "satisfy" me. I guess it would go both ways and I like to think he would be man enough to tell me if the situation were reversed.
I'm stopping here because this really isn't all about ME, but I did appreciate the opportunity to put some of this down in writing for others to consider and to see what it looks like when i read it back to myself.