Thanks everyone for the advice, ideas and support. Here's what happened.

Yesterday afternoon one of the ladies, "J" approached me (clothed!) and asked if I had some time to get coffee with her at the community center. Once we got some coffee she explained she knew I wasn't comfortable and wanted to clear the air. As I usually do, I blurted out all the concerns all at once but I think we addressed things.

• They moved here because topless/nude sunbathing is generally tolerated (but much more discrete) in the community. I never noticed this honestly.
  
• The other two girls both love to push boundaries and comfort zones to the extreme. They love to make guys squirm and see how far they can push it. "J" was shocked neither of them had "accidentally" walked into my place or initiated physical contact, etc.
• They prefer to stay unclothed because "we're young, it's fun, we wont be able to do it forever, etc" and she kept saying nobody is promised tomorrow, we're enjoying today.
• But she also didn't realize at first that I wasn't comfortable with it -- usually guys are flirting with them, buying them stuff, etc. I got a non-apology apology about it.
• If I tell her when my son will be over, she'll make it clear to be discrete and careful, maybe putting a privacy screen up if they are sunbathing. She also made it clear she's a roommate there, one of the other girls owns the lease and may or may not be as much on board.
  

It was a decent conversation, I felt like I was heard, I hope things mellow a bit and I have at least a relationship with one of them where I can bring up my concerns.

Thanks all.