9, come on.
This guy is having problems in the bedroom. He's been a top forever, and he's lost interest in the same-ol-same-ol in the sack with his partner.
You seem to think there are only two ways for this to play out - either he can resign himself to being miserable for the rest of his life with his partner, or declare an open relationship. And if the partner doesn't like the idea of an open relationship, hey, obviously the relationship's over, right?
Seriously?
OP has fooled around a bit on the side, decided he likes certain things, and hasn't bothered telling his partner. That's like deciding you really want shrimp for dinner, then getting upset that you keep getting served chicken or beef, so announcing that it's time to bring in other cooks.
Geez, if you want shrimp, ASK FOR IT.
The OP is in a 35-year pattern. That isn't necessarily a bad thing, but it's fairly easy for people to start thinking that that's the way it is. And it's not the partner's fault for thinking so if the OP isn't going to say anything. Sometimes we need to be alerted to the fact that there's a problem. And jumping straight to the ultimatum is a BS move, I think. If my partner wants me to do the dishes more often, or be more aggressive in bed, or whatever, I'd like him to start with a request. "Hey, can you start doing the dishes every night?" Not "Do the dishes every night, or it's over."
Could an open relationship work here? Yes. Possibly. But it's a HUGE change on a 35-year relationship. It's altering the entire foundation that this relationship is built on. And I wouldn't recommend bringing it in as a "cure to fix all ills" until other, simpler solutions have been tried.
Most LTRs tend to be sexually exclusive, and that's a generally agreed-to thing with most people. You might think that's utterly misguided, that it's gays conforming to a false heterosexual ideal, but I've certainly found many couples who have no trouble with it. And 9, I've said it before - I do NOT have any issue with open relationships, provided that it's something that's agreed to by both parties at the outset. Perhaps you read all the advice above as an indictment to your "lifestyle choice", but it certainly wasn't so from me.
I don't know Dan personally, but I've read enough of him to know that his first suggestion wouldn't be "talk to your partner about opening the relationship". It'd be" talk to your partner about topping you". And I've read enough to know that what works for me doesn't work for everybody else. If being in an open relationship has made you and your partner happy, that's killer. That doesn't mean being in an open relationship is the answer to everybody's ills. If what worked for me worked for everybody, I'd send all the people here looking for a boyfriend to the online video game where I met mine.
Lex