The lack of perceived permanancy of affection may be a motive for lesbian life style. Marriage (man and woman) under 'monogamy promises' may be romantic and wedding vows full of 'forevers", but failure is almost a given. However ladies can predict other ladies that are to be friends forever and notice that affection seems to last longer with a lady than with a man. They have friends that have known each other since high school and will always be friends. They might go 6 months without talking to each other for various reasons, but then after so long apart they catch up and it is as if there has been no time apart. Lady friends forever may sound better than a short monogamous marriage with an agreement already inplace as to how a breakup will be handled legally.
Plural is a whole new ball game though. There is no reason for a loving man to ever leave or stop emotional support and affection for anyone A man and a woman can say "love/friends forever" and in a plural home it can be true forever. A home can be added to without any person finding themselves discarded. A monogamous marriage is not necessarilly bad but "monogamy only" is bad for society and leads to problems.