Learn English – Person who believes that men and women have defined roles

A man (or woman) may be "friend-zoned":

. . .where one person wishes to enter into a romantic relationship while the other does not.

Assuming the man lost his partner, he could be lovelorn ("bereft of love or of a lover") because he is unattractive, unlikable, uncouth or for any number of reasons.

It's often said that "nice guys finish last" or a "nice guy" is "unlucky in love". However, not all who fail romantically are necessarily "nice guys".

As has been pointed out in the comments, it's possible the man is involuntarily celibate. Such people are colloquially referred to as "incels".

Edit: What follows is supplementary but "closes" the question in my mind. I offer few inline citations since the words I suggest are general reference.

First off, I would argue this question falls somewhere between "not constructive" and "not a real question" as per the FAQ. It has clearly created extended discussion and is too broad to be reasonably answered.

Simply put, there is no well-defined term that meet's the question's requirements. Nevertheless, we can examine a range of possibilities and, given better context, one may be selected.

The man is incompatible with some women. Whether it's due to his choice in women or his own traits is irrelevant (and not clear without context). We can add that the man is romantically incompatible with some women to be more precise. Furthermore, we can say, "As far as establishing relationships go, he is a born loser," i.e habitually unsuccessful in his attempts. Again, whether it's because of a social anxiety disorder, negative personality traits, bad luck, poor decision-making in mate selection, etc. is irrelevant although this context would be far more informative than trying to describe the result as the question wishes.

The man may be considered a social or romantic reject ("one rejected as not wanted, unsatisfactory, or not fulfilling requirements"), romantically ostracized, or a social outcast, though there is some dubious circular reasoning here -- but we're not trying to describe or ascertain the why of the situation which is implicit in many alternative descriptions.

The man is the male equivalent of a spinster:

[. . .] a woman who has not formed a human pair bond by the time she is approaching or has reached menopause and the end of her reproductive lifespan.

There is no word with the exact same connotations for males. The closest we have is bachelor, further qualified in unwilling bachelor (see Section (h), e.g.). As a single person, he may be considered "on the shelf".

Ultimately there appears to be no single word to describe all the nuances the question may wish to convey. Sadly, one will have to rely on multiple words (perhaps even multiple sentences!) to express the situation.

After doing some round-about searching (i.e., searching the wiki archives starting with "friendship"), I believe I found the word I was looking for: heterosocial, meaning prefering non-sexual relations with the opposite sex. Thus the man in question (whose wiki page I'm likely to never come across again) was exclusively heterosocial, ascribing to him a fondness (of a non-sexual nature) of only female acquaintances and who, with a dearth of a heteronormative love life, could be perceived to be homosexual.