Oxford Dictionaries:
sem·i·nal
/ˈsemən(ə)l/
adjective
1. (of a work, event, moment, or figure) strongly influencing later developments.
"his seminal work on chaos theory"
2. relating to or denoting semen.
"the spermatozoa are washed to separate them from the seminal plasma"
Northern Light, I dislike that first listed sense of the word at least as much as you do. As far as I know it was never used this way until the early 1990s, when I first started noticing it in places like album reviews in Now Magazine
and The Toronto Star
. It reeks of proto-hipster/wokesters with unusual eyeglass frames, funny haircuts, pinched faces and unearned airs of superiority, sitting at tables in the lobbies outside classrooms at plate-glass universities, who I'm sure started using it to delight in provoking precisely the reaction you (and I) had.
But it's in the ridiculously permissive dictionaries now, which means it's too late to do anything about it except scrupulously never using it. Today writers use it either unknowingly or in order to come across as writerly and more-learned-than-thou, not realizing that they are not writerly, not learned, and actually have a shallow-rooted and impoverished feel for English. But at least once in a while there's a decent and usable neologism, and a perfect example is the one that describes this sense of 'seminal' to a tee: cringy.