I'm inclined to think that it is completely neurological and not blood pressure related (at least, not in the age groups who most commonly use MDMA). One thing to consider is that other drugs which effect blood pressure (such as amphetamines) do not necessarily effect erection. At least, not unless taken cronically.
SSRIs have a similar effect to MDMA in this respect, except that someone must be on SSRIs for a week to several weeks before erection is inhibited. SSRIs do not perform any aggressive actions on the serotonin receptor sites. MDMA is aggressive, and may explain why the effect is almost immediate.
There are several sites (i.e., 5HT1a, 5-HT1b, 5-HT1c and 5-HT2s) that contribute to sexual function. I suspect that once MDMA comes into play, that these sites (especially 5HT1a) are too engaged interacting with the MDMA to perform the functions needed to send messages to the corpus cavernosum (a tissue used to trap blood in the penis and result in an erection) and other tissues involved in creating the cascade effect resulting in erection. Most of these physical responses are chemical (the result of a release of nitric oxide, and others, triggered by the 5HT1a messages sent).
At a lower dosage of MDMA, less 5HT1a receptors would be "MDMA engaged" and would be free to signal the corpus cavernosum to trap blood. And this is also true once someone is coming down and these sites become available again (allowing for erections).