I understand this was during the "angsty playboy" stretch of his timeline and I also understand that he was initially frolicking with her as a cover for his investigation into possible Zygon activity.

But.
.With his abilities he didn't really *have* to marry her Nd just something in the way he looks into her eyes and promises "Ill be RIGHT back..." always made me wonder...

Considering she wanted to take his head off from the start of their relationship, it was always going to be complicated

I'd say yes and no at the same time.

Obviously he went their for a reason at first, then I think, in isolation, without any companion or the rest of the universe bothering him, just the two of them bonding, I think he did start to care and have real feelings. Those feelings I think weren't deep and lasting though, just as I say, a product of isolation from the rest of the universe and his normal life, as proven by the fact that when his past and future selves arrive, and suddenly zygons are showing themselves as a threat, it's back to the thrill of the adventure and the bond that was developing between him and Elizabeth is almost instantly all but forgotten about.

Plus, he must have always kept it in the back of his mind that he couldn't really let a relationship between him and a famous important historical figure go anywhere, and likely just told himself he was having a bit of fun. Then everything happening to break it up reminded him of that fact.

The doctor was very much a playboy during his 10th incarnation though it has to be said. Rose, madame du pompadour, Queen Elizabeth I.

He cared for Elizabeth I as much as he cared for any non-companion human, or the human race in general. No more, no less. I always thought 10 was spending time with her in order to expose the Zygon plot and the romantic interlude with her was just a pretext as he suspected her of being a Zygon.

Should have ran off with Martha ...

Surely DT nicked bits of his Casanova character for 10?