Sexual/Romantic Orientation
Kanda is predominately heterosexual and heteroromantic. But it'd be very easily to mistake him as being asexual and aromantic given his cold disposition and apparent lack of interest. This is in part his repression and resulting lack of understanding towards his own sexuality, in part "that person", and in part a tie between attraction and emotional connection.
Since Kanda doesn't really grasp sexuality very well and isn't educated in the subject, he represses it on a conscious and subconscious level. While he can recognize feelings of love and romantic attraction, he has a harder time with sexual attraction. Thus on the occasion he would experience it, he'd probably just brush it away with little reaction, and his romantic attraction is so unlikely to happen he's not really been in a situation where he's experienced it (aside from That Person).
His almost obsession with That Person actually only helps full this total ignorance and disinterest of other people. While he's only made plans to find her and keep his promise, and recognizes the chances he could even have whatever relationship they used to is little, he still focuses heavily on her to the point that it inhibits an interest in other people. He figures he doesn't need that kind of love and a relationship anyway unless things do work out with her. He has other things to be worrying out, so he pushes the idea aside and just doesn't pay attention to that sort of thing.
And then there's that his physical attraction is very closely correlated with the emotional connection he has with the person in question. Not that he can't find someone he hardly knows sexually attractive if he actually payed attention and didn't totally shove away and ignore everything in relation to his sex drive, but he's much more likely to find someone attractive if he has a close bond with them. And even then he probably doesn't recognize it as possible because he's too busy repressing himself.
Romance and Relationships
Kanda also has only a vague understanding of how romantic relationships work. There's handholding and kissing and time together and after that point he just kind of trips and stumbles through it. And he's not concerned because he figures he'll be alone forever. In the event he was with someone romantically, though, he would put his effort into it and into them (because he's not actually that much of an indifferent asshole to people he genuinely cares about. They're just rare). Not that it wouldn't take a lot of work to get him from the awkward lack of understanding (though he'll never be a Casanova), but he'd try his best with what he does no, because he'd care about them and care about making it work. He just wouldn't really know how to go about it as well and struggle with actually displaying a lot of affection and feelings.
And Kanda wouldn't even consider getting together with, or even dating, someone he didn't love or was on his way to loving. He's generally asocial as is, and he wouldn't be comfortable in a situation that expects intimacy when he doesn't really want it that badly with the other person. He's gone this far without a relationship, he isn't exactly needy for one.
But even those he's not looking for one, that doesn't mean he wouldn't take the chance if he really was that in love with someone, unlikely as it is to happen. The situation they're in doesn't matter to him, because he's not going to let it stand in the way of his happiness if he has the chance. Even with his duty of an Exorcist in his first life and the high risk of death he didn't seem to let it get in the way of having something with Alma.
Kanda's also monogamous when it comes to relationships. This has less to do with finding polygamy wrong (though he'd probably consider it weird since it's just not common or viewed as normal in society) and more the infrequency of developing romantic feelings and the investment into the person he's already with.
He is also more evenly biromantic, rather than predominately het like his sexual orientation.
Sex
Kanda is repressed. Very repressed. He's grown up spending his whole life focused on fighting and finding someone, and living within the walls of a Catholic organization with very poor education. He knows what sex is, and what it means to most people, but when it comes to his own sexuality he's really at a loss. He doesn't have a good grasp on the concept as a whole besides the basics, and so when it comes to his own urges and desires he just shuts it right now. Not that Kanda has a very strong sex drive, but it's there and when it happens he makes quick to brush it aside and ignore it rather than acknowledge anything. His lack of understanding and repression actually makes him very prudish about the subject.
Generally Kanda waves sex off as unimportant. Even in a relationship he'd much rather do things that are romantically intimate than sexually intimate. He's not opposed to the idea of sex in a relationship, but stomping out what little drive and arousal he has makes him not even consider it. He doesn't attach any particular meaning to sex beyond "it makes babies and people who are married do it", and any more extensive view on it is very much under the influence of whatever actual experience he will obtain.
Not that this lack of meaning would make him comfortable with sex with a stranger if it came down to it. Even if he doesn't really care one way or the other, he recognizes that it's an intimate activity, whether that me emotionally or physically. And Kanda pushes people away, not brings them closer--he wouldn't be comfortable in that situation as it stands. As well, there's the concept of vulnerability in it, though this isn't something he'd consider until in the situation itself. It leaves him open for others, and there are very few people he'd trust with the very base of him.