Without naming names because I've been surprised at some other places with there remodeled rooms, I believe the ambience, and highlights such as mirrors or extra views sometimes can add more a romantic feeling depending on day or night views to adjust, sometimes even candles, to me the personality and connection of us means the most, but combined with dim lighting or soft music or even on a balcony can be different ways of sexy and romantically a great time at any location but as mentioned above are nice places, another is Cosmo they offer balcony with view of bellagio waterfall.

Artisan has a somewhat gothic,  artsy feel to it, lots of black , dark reds, art paintings throughout
No casino or parking garage- small bar-restaurant -I think
The standard rooms are okay but the suites are much nicer and spacious
Not bad at all for the cheaper room rates  
Wouldn't consider them sexy or romantic depending on your personal taste

Romantic rooms...

Some of these rooms I am not sure are available for visitors.
But I like the Mandarin Oriental rooms they are very nice.Smaller hotel with no casino but nice/bar jazz lounge very relaxing
Golden Nugget downtown Spa suite 2-story(very nice)
Wynn Villas or Encore suites
 MGM Skylofts luxurious rooms very impressive.
Cosmopolitan wraparound terrace suite of course the balcony is a plus as well.
All of these rooms are very spacious and plenty of room for naughty fun besides the bedroom.

Oyster.com put together a list of 14 most romantic rooms in vegas.
I don't agree with all of the choices on the list but some of the ones I mention above and other people's responses here are mentioned.

http://www.oyster.com/las-vegas/hotels/roundups/most-romantic-hotels-in-las-vegas

With it's walled lined mirrors. To me, it's the best "Standard" room in Vegas. We can discuss suites and I'm sure all the hotels have suites that can knock it out of the park, but I rarely stay at a suite, unless it's comp'd as an upgrade. So it's the Encore....

I personally like.....

The Cosmopolitan is nice-I love the balconies.I especially like the corner suites, where the balcony wraps around on both sides. The Encore or Wynn Tower Suites are very nice and spacious. I also really like  the Elara, Hilton Grand Vacations Club (they are one of the few hotels, near the strip, which has a large jacuzzi with jets. It is located in the bedroom area. Some of the other amenities there are a separate living room w/ a projector screen, as well as flat screens throughout, a large L-shaped leather sectional, kitchen with stainless steel & granite countertops, 1.5 bathrooms. Candles & music always help set the mood &  ambiance, imo-especially some Julie London, Billie Holiday, Nina Simone or Astrud Gilberto