It sounds to me like this is something you need to address now if your relationship is going to continue to be healthy in the future. Have you talked to her directly about it? It sounds like you guys have been together long enough that you should be able to bring up things like this.
Let her know that you're feeling unsatisfied with the relationship, and try to find out how to fix it. Try not to really blame her, though; just ask if there's something you can do to make her feel sexy more often.
If infrequent sex is a deal-breaker for you and she's unwilling to compromise, then you may have to seriously re-evaluate the long-term plans of your relationship.
But IANA sex/relationship therapist…