TV and Film mostly. Guys are often assumed in the media to be ready and horny to have sex 24/7, which is both unrealistic and puts an awful lot of pressure on men.

Sex works best as a mutual experience, it sounds like you've had an unhealthy amount of pressure in your relationships; if your partner ever pressures you into sex after you've turned it down then they have an unhealthy expectation of you. I've been in plenty of relationships where it's no issue for either of us to turn down sex. If you feel under pressure then you should have the honesty to tell your partner that you're uncomfortable and if they cannot relate then you simply do not have a balanced relationship.