What would be the meaning of "Red roses for young lovers, French beans for longstanding relationships"?

One cannot feed on roses. Not even as a snack. From the purely practical point of view, roses are useless.

What the author means is long-lasting relationships require down-to-earth, prosaic food. Or some such platitude.

The red rose has long been associated with love. To take two examples:

From Western culture, it has been believed that this type of flower was created by the goddess of love, Aphrodite. According to the legend, her tears and the blood of her lover, Adonis, watered the ground, from where the red roses grew. It was then a symbol of love until death.

For the ancient Arabs, red Roses also implied love and romance. They believed that this type of flower yielded a literal effect on their heart. In one of their Arabic tales, a nightingale saw a white Rose and fell in love with that it caused the bird to sing. Because of overwhelming love to the flower, the nightingale pressed its body hard to the Rose so that its thorn pierced its heart. Its blood flowed to the Rose and it turned red. This tale was considered symbolic of the intense love and romance, and the flower of red Roses was the symbol of it.

Rose festival

The rose is thus a symbol of love, a token of affection, a profession of a heart pierced by love, a promise of fidelity, a metaphor for the pain and beauty of love. It is easily picked and presented but, as in the synthesis of Arabic and western cultures made by Edward Fitzgerald in his works of Omar Khayam, its beauty fades and becomes lost.

Alas, that Spring should vanish with the Rose! That Youth's sweet-scented Manuscript should close! The Nightingale that in the Branches sang, Ah, whence, and whither flown again, who knows!

Gutenberg

Contrast all this with the French bean, which has no such such romantic attachments. Instead of being plucked, the bean has to be planted, tended and gathered, taking months of careful and affectionate attention. Once presented it must be cooked and presented again for its benefits to be felt. And its benefits are enduring, in fed bodies and good health.

Hence, the rose is a metaphor for the pains, immediacy and hopes of love, whereas the bean stands for the enduring, patient and collaborative nature of love.