keberoxu, the verse may be even more ambiguous than I thought. 'Prunes' could refer to the ladies themselves, shrivelling up as they get older while life (and love) passes them by. For instance, there is an old-fashioned term for an old woman, shrivelled up with age and hard work, growing ever smaller: She used to be called a "Hutzelweib".

I've had a quick look at the rest of the poem, and it makes sense. The earlier verses are all about the girls' abilities and how they display them. I wouldn't think he means to convey that they are actively striving to stay single but rather, that they use the wrong strategies to attract suitors - displaying their housewifely qualities and forgetting to turn on the charm, perhaps.

Incidentally, the page where I read the poem tells me that the parson's daughters from Rückert's hometown did indeed never marry!