If you are looking for a LTR with someone who has a good personality (family oriented, compassionate, understanding, reliable, good conversationalist, low maintenance, sane, etc) it is best to be mildly attractive, about a 7 or 8 on the 1-10 scale.

If you are too attractive you attract the wrong kinds of people. People just using you for validation, money or sex; people who never look beyond the superficial, broken and insecure people who think if they can attract a high enough SMV person then that gives them self esteem, etc.

Also if you are extremely attractive, then that could cause your SO to get jealous and insecure. Which could be toxic for a serious relationship.

If you aren't attractive enough, then even when you find someone who would make a good LTR, they may not find you attractive enough to reciprocate your feelings. Especially if it is someone you don't know well who you need to date more to feel out each other's identities.

I've known people who aren't attractive but who have good personalities. After someone gets to know them, they may find them attractive despite not liking them at first.

At the same time, maybe only 20-30% of people on the singles market have a good personality. And the chances to get to know each one well enough to get them to look past your bad looks is not always an option.

Basically if you are too attractive, you attract the wrong kinds of people. If you aren't attractive enough, many people will not give you a chance. If your goal is just to fulfill your sexual strategy (for men, to sleep with as many attractive women as possible. For women, to marry a guy with tons of money, power and status) then having the highest SMV possible is best. 10 is better than 9, which is better than 8, etc. But for LTRs, being in the 7-8 range is best.