Akira isn't actually a teenager virgin. He was a 40-ish man in his past life who had girlfriends IIRC. Why's he acting like the usual limp dick male lead here?
There are plenty of possible explanations.
A) The fact that he might have had a girlfriend in his past life, doesn't mean he is adept at romance or a constant horndog.
In fact it has been made painfully clear that he focused all of his attention on his work first and foremost.
B) Medisfina is a royal, you don't get to just have some casual sex with a royal, it will have ramifications and Akira knows that.
C) As you pointed out Akira was at least 40 years old when he died, Medisfina is practically a toddler in his eyes ,perhaps he can't get past the fact that he sees her as too young.
D) Akira is actually more interested in Aria.
E) The promise with Aria he made all those years ago, has some short of "binding" power, (and as we can see in the last combat ,they are linked somehow.)that also "prevents" Akira from "cheating" on her.
F)He might be a former adult but his body and brain(as in the actual organ) are that of a teen, his physical being is that of an adolescent kid with all the raging hormones and confusion those brings, now if you add to that his past life experiences and his powers (along with all the responsibility and dangers that comes with) him not being ready to just "jump in bed" with medisfina or any other girl make a tad more sense.
Now whether one of these explanations stand, or it's just the author keeping the "options" for shippings possible...I don't know.
But honestly, not every manga has to be like Mushoku Tensei.