How do you decide what is acceptable sexual behavior?

It fascinates me to watch this play out in the conversations here. Case 1. A husband wants to do something with his ejaculate, as a recently closed thread discussed. Lots of people say "whatever works for you and your spouse", but the consensus also includes an allowance for the wife to opt out. She should have the right to decide that this is too disgusting or degrading to participate. Case 2. A husband wants to touch his wife's breasts during foreplay, but she doesn't allow it. Lots of people way "whatever works for your and your spouse", but the consensus also includes statements to the effect that "this is a perfectly normal act and a decent spouse would work to include this in a healthy sex life". Is the only difference between these two cases social norms? I very purposely chose two options that do not include pain, or third parties, or non-monogamous unsafe practices, or risk of injury, and there are certainly other examples. So it seems we use some other yardstick above and beyond "only those things to which both partners agree are reasonable". Is that yardstick nothing more than social convention? Is oral sex more expected and acceptable now only because the majority does it? Do we really just rationalize what we personally find acceptable as normal, then construct elaborate arguments to justify our position? It seems to me we do, more than we might want to admit.

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