fiancé doesn't want sex

I have been with the same guy for 4 years we're engaged to be married next year and have a baby due in December. For as long as I can remember now it's always had to be me who's initiated sex and it's like he's not interested at all and the last time I asked him to have sex he couldn't even get it up.  I feel like I must be really unattractive and it's making me feel like shit. Also if he's not having sex with me wouldn't he be really sexually frustrated? He doesn't seem to be. I know I am I constantly have dreams about sleeping with other people because I'm so frustrated! I honestly don't know if he's just became unattracted to me or if he's getting some kind of relief elsewhere but how can I live like this forever? It's worse because I'm still very much attracted to him, It's making me feel worthless :( Anyone else feel this way?