First, of course, is infatuation - which does not last. Infatuation is that initial drug-like "high" of a new relationship. This is why one should usually wait until you've known someone a couple of years to marry - it can take that long for infatuation to wear off and for you to see the real person.
Sexual chemistry - is usually a combination of infatuation and pheromones. It can be lost but it's hard to manufacture - that is, if you're dating someone that you feel no sexual attraction to, it seldom develops.
In a good relationship, the initial infatuation is replaced with a deeper, warmer love and sexuality can become even better through more intimate knowledge of your partner.
In all too many relationships though it seems that infatuation wears off, that deeper love doesn't really take its place or sex is neglected, then the couple ends up in a sexless marriage.
Bear in mind though - when most WASs say they've lost "chemistry" with their spouse, it usually just means they are temporarily befuddled by that "high" of infatuation with a new partner.
