Why is it that nobody says that Jane shouldn't have started the relationship with David in the first place

They do say that as well. Some outright say that Jane is sinning by 'unevenly yoking' herself.

, since she has known all along that he thinks that sexual compatibility/activity is very important in a relationship?

All else being equal it is better for people to look after their own preferences than for others to mind read them and do it for them.