Sex is all very nice, but you can't really do it for more than a couple of hours a day. Love, however, can be a 24/7 thing; it's more fulfilling in the long run. And if you're an insecure lonely wreck like me, it's a great relief to feel there's at least somebody out there who cares about you and who you can trust :-)
But love is a strange thing. I think most people are pretty confused about where the boundaries between love and sex are; indeed, it's taken me a lot of thinking to really come up with a good definition myself, and even then it's hardly universal.
I've separated a few different things in my mind. Sometimes I see a pretty girl, and I really want to kiss her (ideally, kissing that leads onto sex, but for whatever reason, it's the kissing that I primarily fantasise about); that's just lust. Then there's people who I think are awesome, in such a way that I feel I can trust them, and I attach great importance to their opinion of me; that's perhaps best covered by infatuation. And then there's people I've grown accustomed to in my life, in such a way that if they were gone, I'd feel sudden panics when I realised that my normal routines and my model of the world had a huge gaping them-shaped hole in; and then there's people I care deeply about, who I cannot allow to be unhappy without doing all I can to help them. The latter two are what I'd generally call "love", and although I identify them as two separate things in my mind, they seem to only ever occur together.
In the context of intimate relationships, the ideal is to combine all four aspects. I think plenty of long-term couples have the latter two, but lack the former two, and they then complain "the spark has gone, if it was ever there". Lust and infatuation are often considered to be a bit "shallow", but they're important - they're what keep the intensity in the relationship.
It can be hard to keep those intense things going, however. After many years together, a lot of once-thrilling things can become routine, and get lost amongst the daily grind of paying bills and washing the dishes. It's a good idea to pay attention to them in a relationship, and don't let them fade.
I'm still highly infatuated with my wife; perhaps part of this is just because I'm quite insecure so I'm prone to latching onto others, but a big part of it is certainly that we're lucky to be quite intellectually compatible. Our academic interests are quite different, but there's enough overlap that we have common ground, while still being able to marvel at the other's expertise in areas we are useless in. As we're meritocrats at heart, both having things we're better than the other at helps us to feel respect and awe for each other - and reminds us that we're great as a team.
And we keep the lust alive, despite familiarity and the fact that two pregnancies and a host of medical problems have made her overweight and crippled. Part of this is variety; we try and be imaginative in where and when and how we have sex, rather than it just being a nightly missionary-fuck-before-sleeping routine. Part of it is mutual enthusiasm - knowing that she's horny makes me horny, too, rather than sex being something we both do because we think it's our duty to the other. Part of it's having learn each other's turn-ons (and tracking them as they change and evolve). I quite like the idea of having rough, intense, sex with sluttily-dressed nymphomaniacs, so every now and then she'll dress up in a PVC miniskirt, and bend over to be spanked and to let me fuck whichever hole I please; she's into being semi-willingly abused by authority figures (the girl's got issues...), so sometimes I pretend to be a priest/soldier/pirate/etc for her.
I see the fact that we're married as largely irrelevant. Getting married is really a legal technicality to make an existing state of affairs "official", with a big party for all of our friends and family to get to know each other (if they haven't already). It's kind of cute to be able to call each other "husband" and "wife", and to sign things as "Mr. and Mrs.", but I'd caution anyone against thinking that getting married involves a real change in their relationship from "boyfriend and girlfriend" to "husband and wife". We may be husband and wife, but we both agree that we're also boyfriend and girlfriend - and we're also friends, too - and we're also individuals. These things complement, not replace, each other.