If this candidate was as competent enough as the others and we've had to narrow down our list, how much weight should his attitude have? Should we give the "Johns" that exhibit similar behaviors the benefit of the doubt or send a canned email rejection straight away?
Particularly in a small company and particularly when the employees filling the job in question must meet with clients, attitude is just about everything. Give me a candidate who isn't as strong technically but has the right attitude and I can usually train them to become a great employee. Give me a candidate with a poor (or weird) attitude, and the technical strengthstrengths may not matter at all.
If a candidate fits the bill strongly everywhere except a seeming avoidance of women, I'd be direct. Something like "We like your technical abilities, but it seems like you avoided every woman here. Our company and our clients are composed of both men and women and this role requires dealing with both. How can we feel comfortable that you will be able to handle that situation?"
Likely this question will elicit the reasons behind what you are observing, and you can make an appropriate hire/don't hire decision.
If I got strong feedback from others on the team (female or male) that this candidate was "weird and offensive" then I almost certainly would pass on hiring, unless I was convinced that it was something that could easily be corrected or was some sort of misunderstanding. Feedback from themy team is very important to me and fit with the team is often of primary importance while making a hiring decision.