AB v LM  EWHC 1234 (COP),  MHLO 139
"I find on paying close attention to Dr P's advice, but also considering the contribution of Dr G, that Lisa does possess the abilities required to lead to the conclusion that she has capacity to make decisions about whether or not to have sexual relations. She is somebody who has been full to sexually active in the past; she has had children; she understands the rudiments of the sexual act; she has a basic understanding of issues of contraception and the risks of sexually transmitted diseases. The area in which she is weakest is her ability to understand the implications for herself should she become pregnant. Pregnancy for Lisa would be an extremely serious state of affairs; there can be no doubt about that. But her weakness in that respect does not, for me, lead to the conclusion that her capacity is absent; it argues for her to receive continued safeguarding and help, advice and explanation as and when the question of sexual activity might become a reality."
Possible Bailii link (not there when last checked, but it might have appeared since 0700 this morning!)