Rose I think a good sexual relationship is something that needs to be worked at to a certain extent as you become more used to each other and at ease with each other.
But, like Bant says, if there are fundamental differences in what people like or expect, or how the other person relates to you in bed (which can be very different to how they are when chatting in the pub), then that could cause major problems in the future. For example, I did have one guy, who I didn't actually sleep with but was planning to, when he mentioned some things he wanted me to do to him that I was really not comfortable with.
I've slept with two people now whilst OD. The first time, there was a lot of faffing about on my part about shall I/shan't I as I was very nervous as I'd not done anything like that for a very long time, but this time I have had no such qualms. Both went well, the first improved a lot with time and I think the same will happen with this one. Both were/are in situations where we both intended to carry on seeing each other, and I don't think I'd be comfortable otherwise.
I agree with the thing about life being too short though. I wasted many, many years with someone who was selfish and rubbish in bed and ended up having no sex life for years whilst we were together. I don't want to miss out on that side of life any more. It's important and a big part of what makes a relationship different to a friendship.