I think the "old religious beliefs" have more to do with it than any connotation of forced sex (at least I have never recognized this in the way people use the word).

"making love" is something partners do as a healthy and (re)productive part of their relationship, "fucking" is what 'animals' do, purely done for pleasure. It is a bad word for the same reason gluttony, excessive alcohol and drugs are a sin. A society where everyone mindlessly chases dopamine from carnal pleasures is less productive, especially when this leads to unrest and violence (and I guess that's where forced sex comes into play).