That is 100% correct. As long as you see "Your relationship with X has been lowered" or whatever the wording is... it means you're doing something against their will (Stepmom/Mom and Stepsister/Sister) but you can slowly get there with serums and patience without doing that. Did it a bunch of times because I've played through this game over 30 times I believe...You do not have to rape Ava to get laid if you have enough patience. It takes a lot longer, but it can be done. Same with Dakota.
Can't wait for updates...