Can't get my son's girlfriend out of my head? This is getting worse?

My youngest son has been dating a 19 year old girl he met through a dating site for a year now, and she just moved in a few month ago. What she always wears is always too revealing and i just cant get it out of my head. I am close to forcing them to move out. Me and her haven't had any connections because i try not to connect with her. I have asked my son to tell her to dress more modestly, but he ignores it. I can't live in a house like this. Our rule in the house is that i don't get to kick them out until they are finished college and they need good grades. My 2 eldest sons have moved out with a good career ahead of them. This is something i promised to my kids when they hit high school. I can't break promises, what can i do? I want him out of the house. I can't kick his girlfriend out because she won't have a place to stay, its a long story...I seriously don't know what to do, i feel like locking myself in my room all day, i feel that's the only way to get her out of my head. What frustrates me is that she has done nothing wrong, both my son and her, but i just cant get these sexual feelings about her out of my head.. I sometimes think its me that is wrong. Maybe i need counseling or something? what should i do?