He’s lousy in bed, I’m tempted to cheat

I am 27 years old and dating this guy who has come to be the love of my life. Our relationship is great, except for one thing, there is no passion in our sex life. Since we started dating, I am the only one who initiates sex. When we have some, it only lasts a few seconds. He wants to marry me, but I fear we may end up living as brother and sister once we are married; with no sexual passion for each other. Three of his former girlfriends cheated on him, therefore, he is too protective of me. I have been tempted to cheat on him several times, which makes me realise that I may be unfaithful to him in future. I love him, but I have my doubts about the stability of a marriage in which sex is bad. I don’t know what to do with his proposal to marry me