maybe i've been looking at your situation all wrong. have you considered the possibility that you're not actually satisfying her sexually? i was in a relationship with a very attractive young man, who happened to be an excellent boyfriend, AND had a great big dick, but he simply didn't get me off. at the start of the 3 year relationship things seemed fairly normal, albeit slow in terms of sexual compatibility, but i figured everything would work itself out. 2 good looking young people, who really liked eachother, and both with active sex drives. unfortunately, it turned out we were NOT sexually compatible, and i found myself doing the absolutely unthinkable: faking orgasms, and then i started to avoid having sex with him altogether. we stayed together for a while, and we were a pretty great and loving couple other than in the bedroom. things finally crashed and burned, and i left him. it was probably about 75% because of the bad sex. in retrospect, had i really wanted things to work out, i suppose i should have carefully guided him through giving me rockin' orgasms, but i had seriously lost interest by the time it was too late.
from chatting with married friends, i do believe it's quite possible for couples to start out hot and heavy, and have things cool off considerably after marriage/kids. unfortunately, the only thing i really know about marriage is that my boyfriend divorced his ex once he lost interest in her sexually, and decided he no longer/maybe-never-did love her. yikes.