[ --more NON-EXPERT response... just adding, hopefully-- ]
Strange, I had low Vitamin D when I was tested a year ago, as well (was the only thing "off" on my tests). I'd been supplementing it ever since, but haven't had a second check-up to be sure... Now, of course Vitamin D relates to a TON of things, so just doing what I'm doing isn't necessarily a good idea (monitoring it as well is key!).
You looked into Oxytocin, too, I see

Essentially, nothing but a trip to the doc for an Oxytocin dose would easily show you the effects of that hormone on sexual arousal (might have to be injection though >_>) .. who really knows, otherwise? All we've got available to us are studies and a buncha internet text that 'talks' about it... which obviously don't really say much for what it'll do in YOUR specific case. Such a thing isn't an uncommon procedure, though (wish I had insurance -_-), and side effects are few to none.
But, in any case.
As far as your last paragraph, I'm in no position to say or assume what you may have or may not have.

BUT-- never forget that anyone with impaired parasympathetic drive will show results akin to "chronic stress" because the sympathetic system overcompensates (and uses those non-pleasurable stress hormones to signal, impeding erection, though being necessary for orgasm--but not for the sensations during orgasm).
Best of luck...