"It is still a widespread expectation that adult relationships are exclusive at a given time. I think that is a reasonable expectation"

Odd.

I believe that enlightened people understand that consent requires assent, not assumption nor expectation.

Unless you have been told that the other party desires fidelity for their part expecting it is highly unreasonable.

Heck, it used to be assumed that "going steady" should be reserved only after a long time of dating around, and only for the most intimate relationships.

I still find this to be true; that being chosen as "the one" is about as far from the default expectation as is humanly possible.

Maybe females in your head behave as you expect. But out here in RL, you need to listen and respect choice, not rely on expectations, nor believe that others are being unreasonable when they have different expectations from yours.

That sounds like a formula for frustration at best, as well as being a bit rapey.

Dante