Flygirl, no need to apologize to any member here. Anyone here is free to speak their mind, as long as no personal attacks or abuses occur. Some persons here are more blunt or briefer than others, but of course they run the risk then of being misunderstood or having that bluntness returned. You were very fine in your posts, so no worries. If anything, you are overly thoughtful. Likewise, I am usually that way, too, unless I start to get really bothered at something or someone.
I think what makes your situation with your neighbor difficult is you seem to have put forth many efforts and let your guard down, after he showed you that great interest and ability to have a great side and with several relationship capabilities, but then you mentioned he abruptly turned those feelings off, and suggesting he gave no good reasons. In a way, it is contradictory that he showed he cared much, but then shows distance or apathy. That confuses you, and rightly so.
Although there have been some similar thread scenarios in this forum, where the NT was getting or feeling very close to the one with ASD, but that person with ASD could upon feeling pressure, confusion or sense some critique quickly go away, or with them keep returning at their own best time, there are many others with ASD who could be the ones rejected, with the NT being the one who is causing that. Each case will be different, as each with ASD will differ, and each NT or other person with condition will differ too, as comorbidy occurs too.
The success would seem to depend on many things: how each handles stress, their relationship expectations, their communication abilities to the other, the philosophies and interests of each, and the other abilities, limitations and needs of each. And yes, empathy can play a key role. If either lacks it, more misunderstandings and problems could occur, if one is always giving, and the other more needing or taking. In your situation, I see both as giving efforts and having abilities, so I see some hope.
With regards to compartmentalization, I think anyone can compartmentalize, regardless if they are an Aspie or with ASD, or an NT or with other condition, and that is not always wrong. Let's say one comes from an abusive household with abusive parents. It could be instinct for many such persons to become emotionally detached to protect themselves from further harm by not trusting and withdrawing. That is a defense mechanism everyone has to bring some safety or comfort to their lives.
A problem though could occur if that person generalized that all parents were this way, or all men or women in society were that way, so they then acted this way to all persons. For Aspies or ASD persons, or those with other conditions and traumatic upbringing, we would have no way of knowing if any parenting caused them to emotionally detach, or genetics. From reading many posts in this forum, I just know ASDers can express certain emotions differently, or more, or less, or a switch can seem to be turned off at any moment, when emotional closeness needs to occur.
For your situation with your neighbor, my guess is he did all the many things he could for you to show he cared, as he sensed or knew he had limitations that you would eventually find out about. And so once he started showing less sexual diversity later on, either because he was losing interest in that or was wanting to show more of who he really was instead, maybe his mind told him to detach himself again for fear of being rejected again.
That is just my opinion based on the few posts from you in this thread I have seen, as there are other possibilities as well that could explain things. The only way to know is to try to talk to him more, if it is your and his desire, and both share your feelings more too. Of course any opinions here could change if new information comes available that would lead in another direction, as again each with ASD will be different. Some could be asexual, desiring sex often, or only under the right scenarios, etc.