Does your mind ever wander from sexual thoughts during intercourse? Mere physical touch isn't enough to keep a man aroused for the most part. If you're not in it mentally, you can lose your erection quickly. (This mental prowess is also partially why some men can come on cue.)

Just try to keep your mind in it. Keep thinking about her pussy, ass, tits, etc. Think of how good it feels. That kind of thing.