>>56975452Thistles are in the same clade (Eudicots) as currants, but that's not a particularly close connection genealogically.
There are a couple of reasons I can think of as to why it might be appealing, though I'd say it's something that depends on an individuals taste.
For one, people can use the male interest as a vehicle of experiencing the fantasy of being fawned over by women. For another, people have always been interested in status. Vintage wines, overpriced cars and furniture... having something that other people desire but do not have themselves is one hell of a drug. Of course, considering your SO as a possession isn't the greatest thing to do ever, but people still do do it.
Then there's me, who loves conflict and sees "to ensure the continuation of love" as the best possible of all casus belli. 