I think we have to look at this from 2 angles. The first is that this is a porn game and therefore, despite things like its good story that make it more than just a regular, run-of-the-mill porn game, it does still follow porn logic a lot of the time and that logic says that Maya can indeed take the MC all the way with little effort. She's also clearly not a virgin and I guess we have Josy and "Mr Dildy" to thank for that (would have been nice if the MC were the one to pop that cherry, though
), so that may account for the lack of discomfort, and yes, he's certainly bigger than average, but again, porn game. ¯\_(ツ)_/¯
Secondly, she was hesitant to do anything too risqué, but that was before Josy arrived, and I think having her there, having her encouragement, allowed Maya to be more bold. So she has the encouragement and support of two people she trusts and cares about, plus we saw how encouraging Josy was for Maya to have sex with the MC at the start of the episode, so despite the circumstances, Maya clearly feels braver because of this and I think we're seeing the kind of positive influence Josy can have in bringing out this side of Maya.
As for that scene, I do agree that I would have preferred a more intimate and special moment for this first time, but at the same time, I think it serves a couple of purposes. The first is that it gets the Scavenger Hunt done quickly and easily. They could have taken more time, had a proper threesome, but that's probably something they (by which I mean M&J) would rather do privately, so all they had to do here was make it look legit. Secondly, it gets Maya more comfortable with the idea of sex with the MC since she knows she can take him which means she'll be less nervous and more excited about it the next time. Josy seems to be good at breaking down Maya's inhibitions, and this may have been part of that. Finally, it could make the potential "birthday present" threesome more special because they all got a taste (pun partly intended) of what this relationship can be like, so there could be real desire and passion in it rather than trepidation and uncertainty.