I could be accused of the first option, but it depends on what you mean by "long term", I've had 2 that lasted around a few years, others that were less and some that were only a few months.
It's just not happening and I'm still in no rush for it to happen (age 29) but I don't think I have the inability to make a long-term relationship last, in the situation you gave, if this guy consistently can never keep a woman happy with him for more than a few months, it sounds like he's just unbearable after a short term. And in a relationship, when you're with someone for half of your time, even a month is a lot of getting to know someone.
I actually know someone (hardly a "friend" though) who sounds just like the guy you described, but his current relationship has been going over a year. He is a good looking guy, mid-30s, and has said before this relationship he just went from woman to woman, probably partly for sex with different women, but I would say mainly because he's intolerable. Incredibly cocky and self-centred, makes usually very poor jokes where he bursts out laughing, and I've seen him attempting to flirt with women and he genuinely seems to scare the **** out of them. It's the most awkward thing I've ever seen, there's no flow at all to conversation and he just jumps in with random questions or laughing at literally nothing.
Would some of them still have sex with him? Yes, would they go out with him? Maybe for a very short period of time. I have no idea how his girlfriend deals with it unless she's also nuts.
Anyway, you're asking for a choice to be made here, and I definitely pick inability to get casual sex. If it wasn't for casual sex I'd probably have knuckled down to an actual relationship. Even the best looking guys who pick up women easily enough spend a lot of time alone, it does affect a big chunk of your social life, for example you never really express yourself to relatives or work colleagues when you talk about what you do with your evenings or weekend, unless you have an official girlfriend it's pretty awkward to just mention you went for a meal or watched a film- with some girl you're banging.
Ability to get casual sex is not a constant strain of quality time with a partner and also sex, it is some limited time, maybe even of better quality (someone new is always more exciting) but 0.001% of guys have the opportunity to constantly bang new girls, keep them happy and never pissed off that it's obviously not going to be serious.
But ability to have a long-term relationship is constant quality time, sex, and being with someone who will potentially always understand you. You are the same as me zircon in that there needs to be more than just physical attraction to really enjoy sex, and half the point of enjoying sex is getting off on the other person enjoying it, if you don't give a **** about that other person then physically the sensations are somewhat as good, but mentally it's just visuals, may as well put on some p**rn alone.