OP, I don't want to judge you or your religious beliefs but you need to leave God in the background and take a more realistic approach to life and your decisions. What I see in you is what sometimes happen to people who are afraid of life and afraid to take control of their decisions. They throw themselves into religion and or anything else that will give them a sense of direction...anyway your issue is not sex after or before marriage (this is not a major factor in determining the long term potential of a relationsip...at the most having sex before marriage could help check physical compatibility which is paramount in a relationship...religious people argue not but Nature or God created us this way, with more or less compatibility depending on the partner...)..
Anyway I'd say you're not happy with this guy (how can he have 3 kids already? and how could he afford a second family if you one day decided to have kids too...is he a millionnaire? So you see, you need to think about all those things before wondering about such trivial issues as a virginity vow before marriage...Believing in yourself is gonna help you more than beliveing in God so it's fine to have a faith but do not forget that you are in control, you decide to be happy, you decided what is right for you, you keep the power. If this guy is only good for one thing: sex then enjoy it with him, take the pill and remain independent. But I can't see a happy marriage for you two.