I believe one has to truly know themselves in order to make that call. In our society we tend to downplay the importance of sexual compatibility when it comes to love and marriage. In fact we are told not to put much stock into it since it will fade overtime. Anyone who has the guts to say it’s "very important" to them is generally belittled or called shallow.
My take is a little different. I believe life is a personal journey and each of gets to decide on what is important to us as well as what our “deal breakers” are. The truth is it is our (romantic/sexual desire) for our mate that separates "relationship love" from (parental, sibling, and friendship love.) Sexual compatibility determines how often the couple does it. If one person would like to do it 4 times a week and the other is happy with once a week. In a 52 week year you are talking about (208 times vs 52 times). That could be a major problem.
Generally speaking it is much easier for people with no sexual experience or very low sex drives to live with "bad sex". The people who have the most difficult time are those who have had mind blowing experiences in the past. They are likely to have flashbacks of previous lovers or fantasies which could eventually cause them to be tempted to venture outside their otherwise perfect relationship in order to fulfill that need. Logically it seems like the easier thing to do when compared to walking away from a great person. Nevertheless it’s wrong and unfair to cheat or betray them.
Once the two of you have tried everything you can to improve things you have to ask yourself; "Can I be happy or satisfied if things remain this way for the rest of my life?" If the answer is “no” then you are better off letting them go.
Hopefully you both will find other good people whom you are naturally compatible with.
Awhile back I wrote a hub on this very subject. Sex: Is bad sex a “legitimate reason” to end a relationship? http://hubpages.com/relationships/badsexendsrelati...