Everyone knows that remain would win any referendum with May's deal Vs remain, or customs Union Brexit Vs remain. Those MPs who say they'll back the PMs deal as long as it is then put to the people are just cancelling Brexit in a way that makes them look like they're respecting the original referendum.
I'm not fully convinced that Remain would win such a referendum, though I think there's a very good chance that they will.
But in the end, the 2016 referendum was flawed because it put a definite option (keep the status quo) against a phrase 'leave the EU' that actually encompassed a whole variety of completely different outcomes. I'd say that there's a good argument that a 2nd referendum putting Remain against a definitely known version of Leave is perfectly respectful of the 1st referendum: It's basically saying, "
52% voted to leave last time - without any clear idea of what 'Leave' meant. The Government has as a result spent 3 years coming up with the best version of Leave it can work out. This is it. Now that you know what the most deliverable/most favourable version of Leave means, do you want to go ahead?" If that means that Remain wins - then we'll know that more people wanted to Remain than wanted to proceed with the most deliverable Leave the Government could come up with.
I don't see a problem with that [*]. Do you?
[*] Other than a general issue that, in my view, this kind of highly technical issue is really not suited to a referendum. But that particular cat is already well and truly out of the bag.