I know about it.
I can be so anti-sexual-first that I originally assumed I was sx-last. I do everything I can to repress my sx-first needs from consciousness, and other people have suggested I'm sx-last as well. I'm not, I'm just virulently anti-sexual-first.
For me, it's not so much that I "avoid" intimacy as I am in total denial that I could ever need such a thing. Relationships are for wimps, I have no sexual needs, and you should really get a room. PDA-indulgers should be shot. I don't CARE about your homosexual agenda. Etc. So, I thought sx-last since I have such an adverse reaction to this stuff.
Later, though, I began to see how much my life really has been dictated by wandering, experimentation, and me following my own individual passions; how readily I overlooked my physical and social needs; how obsessive with my interests I truly am. Then I read something about how sx-firsts are painfully aware of the "attractiveness/sexual desirability" scale in any given situation; how they envy those with better relationship-building capacities and those who are higher on the "hierarchy"; how this can cause self-esteem issues and envious torment...and thus I saw the handwriting on the walls. My neurosis was pegged!
It was like pouring alcohol on a wound--pretty much my entire life, I've been stuck thinking I'm some sort of hideously undesirable thing, painfully aware of how I came out on the bottom of every hierarchy, how no one would ever want me, and otherwise creating a self-fulfilling prophecy. Yet I HAD to suppress this from conscious thought--and certainly from the people around me--because doing otherwise would bring too much pain, inferiority, and shame to the surface. I did this by virulent rejection and adamant hatred of everything listed under most sx-first descriptions. Then I'd go home and dwell upon why I was so deprived and my life so unfair. But no one ever saw this; indeed they thought I was a man-hating prude.
Generally, then, the "anti" part revolves around sexual/desirability issues, but can also revolve around intense experiences and who gets to hang out with awesome people. I HATE people who got a better tale to tell about life than I do (vain, much?) and feel less-than by comparison. I resent it when the "awesome" people all get taken by folks more awesome than me. So, I can swing around being very avoidant toward areas where I feel my experiences aren't up to snuff or I'm sure I'll be rejected by the awesome folks (Note: NOT necessarily the popular folks), similarly to how I do with the prior issues. "I didn't need that junk anyway!"
My self esteem really does rest on me being desirable, having intense experiences to share, and being able to interact with those I find extraordinary in some way. If I am denied that, I take it out on myself and avoid these situations to avoid further exposing my shameful lack of instinctfulness.
I wish I could give advice on how I "worked my way out of it", but it's more or less my Big Issue in life, and I doubt it will ever be gotten over, especially now that I'm 30+. This isn't something I do "in a bad mood", this is a permanent orientation. ( I'm always slightly skeptical of people saying "I'm social-first cause I get, like, really anti-social when I'm unhealthy". Sure you do, kid. That's called being PISSED OFF DURING A BAD MOOD.) A warped instinct is extremely painful and, as near as I can tell, not all that easy to work out.