Realistically, the sole purpose of sex is to procreate, because the sole purpose of life is to procreate. You may have a higher calling or purpose or meaning, but that calling or purpose or meaning is predicated on being alive; continued existence of the species is a prerequisite for anything. Even if you as an individual organism do not view the continuation of your bloodline as a benefit, as a component of the human superorganism you have an obligation to further genetic diversification and selection for the good of the race. Unless you are knowingly dysgenic or antisocial, you have an obligation to procreate, and any relationship that does not further this obligation is a waste of time and energy.
Inbreeding is universally regarded as bad for the species. Thus an incestuous relationship that procreates is bad. But barring some outliers, a voluntary failure to procreate is also bad for the species. Thus an incestuous relationship that does not procreate is bad, as it either precludes procreation entirely or wastes resources that could further procreation.
You could argue against procreation on the grounds that you're dysgenic, and that's valid, but unless your partner is too, you're just wasting their resources. You could argue against procreation on the grounds that you're antisocial, and that may or may not be valid, but unless your partner is too, you're just wasting their resources. You could make the argument that sex serves to strengthen a romantic relationship with someone with whom you desire to procreate in the future, but in the case of an incestuous relationship that's a moot point.
And this is all before acknowledging that familial relationships are irreplaceable and even broaching the subject risks damaging that relationship, while there are, what, two to three billion other potential partners of viable age to choose from if you're not bisexual, most of whom don't necessitate compromising an existing relationship.
The only context in which it could be argued to be a positive is one in which both you and your relative have a very good reason to not breed, and neither of you have any other viable partners who also have a very good reason to not breed. And at least one of you is sterile. And you're confident that a romantic or sexual relationship will be more valuable than your existing familial relationship. And will either last forever or not have any negative repercussions on your familial relationship when it ends. I guess.
