Why won't my parnter make love to me? ?

I'm 20 weeks pregnant and I've noticed the last few weeks my partner won't make love to me. He starts foreplay then he stops so I roll over and try to sleep. But the last few times when he thinks I'm asleep he sneaks off to the toilet and pleasures himself I know this because I can hear the volume of what he is watching. I think it hurts more because he is watching porn. But why are male not sexually attracted to their pregnant partners? What's so unattractive about it ?