In romantic relationships, esp ones that you want to go on long term, and esp 2nd marriages, if either of you values sex, there needs to be chemistry and frankly good sex.I guess I just have a different view.
If I love someone, I take the entire package.
When I met my wife almost 28 years ago, my attraction to her was based on HER, not my previous fiancee that I had extricated myself from. I have never compared any woman I have been with to any of their predecessors. They all stood on their own merits.
Maybe I have been lucky, but chemistry outside the bedroom has always transferred inside the bedroom. The key to that has been communication. I have always given my best efforts to please the person that I have been with and they have reciprocated. Does that mean it has been perfect? No. I'm sure that the women that I have been with would say the same thing. Is any sex perfect? I doubt it. However, that all factors in the entire package.
We are all a sum total of our experiences in our lives. No one has a crystal ball. Some of the best learning that I have had has been a blast because both myself and my partner have turned our freak on. I have shown women what I like and how to do it and vice versa. It has not been an issue because the presentation was in a non threatening manner and was in the context of getting off. If there truly is chemistry, respectfully showing ones likes should not be threatening.
My wife and I are polar opposites. Chemistry is what attracted us to each other and what has kept us together through the good times and the bad.
OP had really good sex with her ex, but her ex has moved on. My point is that the problem may not be the message, it may be the delivery of it. She either needs to be dedicated to building a sexual relationship with her BF (and there is give/take in any relationship) or be fair to him and move on.
Some people do not value sex, god help them IMO. Some people don't really understand what good sex it, My Fiance did not. I am not the most handsome, I don't have the biggest equipment, but I have been around and I have more than a little experience. She did not really understand multiple O's and a lot of positions, she did not understand a lot of things, but she does now.
Now before any of the above we had chemistry, and I kind of think love at first site, but maybe it was lust at first sight, I don't know.
What I am saying is, if you cannot tell/teach your partner what you need/want and the sex is not really good by default (Chemistry/experience) then it will cause problems later in the relationship.
In OP's case, she does understand what good sex is about and odds are the after a couple of years, at some point she is going to want to have her brains banged out, and if her BF can't/doesn't figure it out, it will become a problem.