How to talk about sex with someone so defensive?

[NightRabbit]

So, I'm going out with this guy for a couple years, him being a few years my senior. Well, about ~15 yr age difference. I'm 25, he's 40. He's a good guy, and I know he cares about me, but he's notoriously bad at anything resembling criticism. He'll get very defensive and insist that he's done "nothing wrong". This is a repetitive conversation any time I want to talk about anything more I may need out of the relationship. His stance becomes very defensive and he spends more time justifying and explaining why he does what he does as opposed to saying, hey, if it matters to you, I'll try to do X differently.

Therefore, I tend to pick my battles and sidestep confrontation. I try to positively reinforce behavior that I desire as opposed to nagging or "let's sit down and talk about this". This works well for the most part.

However, we need to have a discussion about our sex life. I'm sure he's enjoying himself but I'm not. He was a lot more adventurous and aggressive in the earlier stages of our relationship but things have slowed down quite a bit. Now he's very lethargic and hardly aggressive. He never seems to expend a lot of energy on me, hardly ever gives me head- in short, he seems to be getting lazy. I am as attractive as normal, and he gets hard upon the drop of a dime, so it's nothing like that. In fact, he wants to have sex a lot more often than I do, so I doubt it's an attraction issue on his part.

I tried talking to him about this before, simply, "I wish you'd go down on me more often". His response was an uncomfortable and defensive, "I thought I did go down on you!" without looking me in the eyes, and then a reaction to the extent of "oral sex should be something that happens naturally, not out of a sense of obligation" and now I've made him insecure and me insecure about it and I'll always wonder if he's doing it naturally or if he's only doing it obligatorily and our sex life is now going to be worse from overthought and maybe this wasn't a good conversation to have.

Anyway, that was a little while ago, and things haven't really improved. I don't know what to do. I don't know how to bring this up again since the first time went so badly. But I also don't want to carry resentment around- it's only making things worse, since now when sex starts up, I think, "what's the point of this, I'm not going to have any fun anyway" which I know is unfair to him, and defeatist and counterproductive, but he rarely proves me wrong. Even when I start things up, he seems to put no energy into it. I come out of our sessions feeling vaguely cheated, esp. because I am very enthusiastic about giving him head and getting into things, etc.

I don't want to end a good relationship on some stupid sex issue, and I know he certainly doesn't, either, but I don't know what else to do. How to talk to someone so reticent to discuss these sorts of things?