Long term relationships are different from early dating, you have your own communication and boundaries with your long term partner that are really different than when you are first starting out. However, domestic rape is real and yes you can totally sexually assault your partner and you do not have automatic consent just because you've been in a relationship.

That said, this book advocates doing physically violating things to all women you would date in a short term time period, which is the kind of thing a serial rapist does.