Neither PEVO nor Heidi Priebe lists Sexual/Social (Sx/So) as a common Instinct among INTPs – a point with which I differ. I suspect that Instinct is misreported nearly as often as Type.
Sexual/Socials “channel their sexual energy and aggression into intimate, one-on-one relationships or into heartfelt political, artistic, or scientific causes.” I think we eventually discover that the people to whom we are most intensely attracted are not our ideal partners for all situations. That's probably a good thing. It forces us to branch out.
“Heartfelt causes” makes Sx/So a strong preference. I am reminded, though, of Joan Baez's song about her relationship with Bob Dylan, in which she writes “A savior's a nuisance to live with at home.”
At home, some Sx/So's will seek out the balancing influence of a Secure Primary or Secure Secondary. Out-and-about, a Social Primary can force us to get out of our own heads and interact with other humans in a more casual way. These pairings come with their own tensions, though. Each partner is being pulled way outside their comfort zone and is likely to have friends and associates with whom they can share other interests, sometimes intensely. Earning one another's trust and respect is absolutely essential.
Secure/Sexual (Sp/Sx) is probably my second preference. This is not one of the three that PEVO suggests are most common for us. PEVO lists INFPs among Sp/Sx's, as does Heidi Priebe (though perhaps only 22% of INFPs. For all I know, they use the same source. For all I know, they are the same person.) INFPs may appeal to me for some of the same reasons as Sp/Sx.
So is Instinct or Type the main attraction? I think a strong case can be made that Instinct is a better indicator of similar social motivation, while Type is a better indicator of similar communication styles. Similarity is not the same as attraction. Similarity is not the same as compatibility.