Sounds to me like he wants to keep a relationship with you going while he sorts things out and asserts his independence at the same time. It's probably part of his MLC to create for himself a feeling that he's "dating" and young again, and that possibilities are limitless, etc. I think the "until death do us part" thing is scary to some people, especially when it seems the "death" part is closer than it used to be.
It's my theory that an MLC can be avoided in some cases if the couple are sexually adventurous and explore new things with a long-term spouse, just like you did when you were younger. But if nothing changes, or there is no activity, then the only part of that vow that hasn't been fulfilled is the "death" part. If you see what I mean! So I see the MLC as a reaction to that "deadness" in a marriage, with the man thinking, "god, is this all there is going to be for the rest of my time, and then it's death -- no way, I have to shake things up!" Just my impression of an MLC.
Using Viagra while not dating? Well, LOL, the experts recommend that a man try Viagra by himself at first to see how he reacts, and just go solo for the first few experiences. Or is he a long-time user already?
There's also a joke in there about how can a man possibly know when to take Viagra, because only the woman knows if there's going to be any sex? Having never dealt with any of these ED drugs, I can only imagine the frustration of popping an expensive pill, only to find out the woman wasn't in the mood. Oh, the thought makes me sick, actually, having dealt with so many rejections from my wife over the years -- and the thought of adding the waste of an expensive ED pill on top of that, and then being turned on and having an erection, and then being told "no" again. Ugh!
So if a man with ED takes Viagra on the mere hope of getting laid, isn't he putting himself under a lot of pressure? Never thought of that. But who knows what it means in your case. Does he also carry condoms?