Basically, it is making something desirable or acceptable when an undesirable situation is presented. A rose with thorns intact is considered unacceptable as a gift, but the florist ''stems" the rose, e.i. removes the thorns to thus create a very desirable product.
First we stem the rose, then we place it in the flower arrangement. A dozen roses, stemmed and wrapped, is only 39.99 while supplies last.
Its a euphemism for "struggling against love". In the movie "Brokeback Mountain", Jack and Ennis were hired to protect the sheep from wolves.... ie... to _stem_ the wolf attacks. Instead of stemming the wolf attacks, they stemmed the rose... which meant that they were battling against homosexual desires. Essentially, it was used to equate _being overtaken by same-sex-attraction_ to _being overtaken by a wolf attack_ .... and was used as a device to show that the boss saw them as being attacked by love.
Despite being from two families at war with each other, Romeo and Juliet were unsuccessful in their attempts to stem the rose... and love won out.