My garden is a haven. On a languorous evening, strolling among favorite plants, listening to the mockingbirds, or the great horned owl, it can give me a great sense of peace - and romance.
Not all these factors can be controlled. The owl only visits when he is in the mood, and the plumeria bloom when they feel like it. Sometimes there is no wafting breeze.... and the moon is occasionally hidden by clouds. 
But many factors can be determined by me, and to make a garden ‘romantic' does not require a lot of work. I personally associate ‘evening' with romance so my garden is geared to be enjoyed in the evening.
Consider what we need:
An inviting space, peaceful, and with aspects that appeal to the senses.
We want a space to sit, a place for two....a lovely old wicker or wrought iron bench for instance, or a rocker with floral cushions placed strategically, maybe facing the setting sun. Put a small table nearby, for a place to put your glass, or a candle. Nearby could be a water feature, a little stream that trickles, or a pond or birdbath that will invite the songbirds to visit.
Behind the bench, or maybe over it, I see an archway, arbor or trellis supporting a vine or hung with orchid pots. A sweeping tree with blossoms which overhang.
A shepherd's crook with hanging baskets overflowing with trailing plants. Then a path that meanders and disappears into a dark space.... A statue could be placed there, or a piece of very individual garden art. A small windchime, a gazing ball to reflect the moonlight.
Our space should be away from the road, private, even secluded, shielded from prying eyes by a hedge, a few fragrant bushes or a fence. The word ‘fence' does not immediately conjure up ‘romance', but with some artful touches, maybe painted a restful color, some hanging pots, a cosy corner can be created.
Consider plants with foliage that catch the moonlight. Glossy dark leaves will do this, as will bright variegated foliage, and the aptly named ‘moonflower' or other white flowers. Obviously any night blooming plants are good choices, especially those as dramatic as night blooming cereus, or - with the added bonus of fragrance - night blooming jasmine. Butterfly ginger truly can look like eerie ghostlike butterflies in flight in the dark....
Hang some lanterns in the trees, to be lit at a moment's notice, or place candles in mosaic glass holders or terracotta pots on the ground. Light up a palm tree's trunk with a strategically placed light fixture.
Maybe a favorite flower has just come into bloom..., or you have a story to share about your day. Plan a regular visit, or just spontaneously decide to go for a stroll together. Whatever you do - when you enter your romantic garden, enjoy the mood, be unhurried, plan to linger, let your senses be filled with the impact of what you have created.
Here is a list of suggested plants (geared to my zone 10, but obviously similar plants can be chosen for your particular climate):
Night blooming jasmine (Cestrum nocturnum)
Night blooming Cereus (Epiphyllum oxypetalum)
Moonflower (Ipomoea alba)
Roses - especially white.
Orchids
Butterfly ginger (Hedychium coronarium)
Palm trees
Sky vine (Thunbergia Grandiflora)
Honeysuckle (Lonicera japonica)
Plumeria
Sweet alyssum (Lobularia maritime)
Dianthus
Lilies
Bleeding heart (Dicentra spectabilis)
Wisteria
(picture of garden bench courtesy of overstock.com)
