Questions about masturbation, birth control, premarital sex, homosexuality, nonbinary gender, and even open relationships need more than the unsupportable claim that “The Bible says it’s sinful”… because time after time the evidence does not support the mainstream conclusions. All these things can be sinful, and so can married, heterosexual, reproductive sex. The Biblical teaching about sexual sin does not draw the same lines as the dominant Christian traditions – whether conservative or liberal.
We cannot just dismiss the problem of sexual sin. It just looks completely different than what I expected when I set out to study it!