There are other reasons than specifically wanting to have sex for the sake of having sex. For example, some asexuals want to please their non-asexual partners, and it isn’t necessarily uncomfortable for them, they just don’t specifically desire that particular action. Like, asexuals can still dig the feeling of sexual stimulation, since it is meant to be pleasurable. It’s like, if there’s cake on the table, but you don’t particularly want cake at that time, or you don’t really care for cake, you can still choose to eat it because even if cake isn’t your favourite or you’re quite full already, it still tastes fine, and your best friend is eating some with you.
Alternatively, it’s possible to sort of “trade favours”; instead of sexually pleasing their asexual partner who is disinterested in receiving sexually, the non-asexual party might cuddle, kiss and otherwise be physically affectionate with their partner while making love. This’d still count as sexual intimacy, even though only one person in the act is - hopefully - orgasming.
Some asexuals see sex as a way to bond with their partners. Again, the act isn’t specifically interesting to them, but it works as a means to an end - through it, they get to feel closer to their partner.
Asexuality is the absence of sexual attraction, the “I want to bang that” trigger that most people have in response to potential sexual partners. Not the absence of sexual activity itself, which would be celibacy - the choice to not engage in sex, the act. An asexual can have sex and not feel that specific type of attraction to their partner, as attraction is passive and not based in active choice. It doesn’t mean that that partner is disgusting to them, either; they may well be aesthetically attractive to the asexual person (the same way you can appreciate a beautiful statue or a painting without becoming sexually aroused by viewing it) and the asexual person may well desire physical intimacy with them, such as hugging, kissing, cuddling and holding hands. Since not all asexuals are inherently repulsed by sex but merely disinterested in it by default, turning that desire for general closeness into sexual activity where it provides fulfillment of some form to both parties isn’t necessarily paradoxical.
Someone once brought up a fairly good point regarding this subject; non-asexual people also sometimes hook up and have sex with people that aren’t specifically attractive to them. It can still be perfectly consensual and satisfy the needs of both parties, despite the fact that the base attraction wasn’t there. For example, a person who just wants to have sex with someone might go along with a partner they would not otherwise choose, but who simply happens to be available and ready to do the deed with them. Alternatively, a woman might seek to have a baby, and have sex with a partner solely for that purpose, and whether the partner chosen is sexually attractive to them or not isn’t a big factor in the choice. (Plenty of asexual women choose to have sex to have children.) There are multiple reasons to have sex even when you’re not sexually attracted. Specifically for asexuals, the factor of having a non-asexual partner is usually a big motivator to have sex, and other reasons like the ones I explored above may additionally pop up to support that decision.
Finally, some asexuals just really dig orgasms. Most people do. We just don’t have that special someone we wish was delivering them to us, and largely prefer to take care of our own. But when you are in a relationship and that kind of comes as a package deal, some asexuals don’t feel like it’s a bad trade, or at least aren’t violently opposed to the idea. I think most asexuals who do choose to have sex are indifferent to it - it’s not their favourite thing in the world, but it takes care of business.
