To start nitpicking: not all Rosacaeae are guaranteed to have perfect
flowers. It is the rule but exceptions can be found.
My local flora states that Sanguisorba can have 'male' flowers only
PvR

Monique Reed schreef
That's not a case of unisexual flowers, though, not in the Rosaceae.
It's self-incompatibility. With things in this family (notably
cherries), often one cultivar is incompatible with itself and needs a
different cultivar. You can get around this sometimes by grafting on a
branch or two of a different cultivar.

M. Reed

P van Rijckevorsel writes
Isn't there any number of fruit trees who need to have a 'male' planted
somewhere near them for them to bear fruit? Maybe I can think of them
when feeling a little less woozy.
PvR

Stewart Robert Hinsley wrote:
Apples for a start, though I wouldn't swear that it applies to all
cultivars. Sample URL -

http://www.ontarioprofessionals.com/trees.htm
--
Stewart Robert Hinsley