To be honest, I've always found serial romantic one of the easiest aspirations in game. But I've never tried it while a sim had a spouse though.
The simple solution is building a nice bachelor pad on the bluffs and just have all dates over there, doesn't even cost money.
If you do want to spend money, I think you can do the same by buying a retail lot and just never open the shop. You can have dates on retail lots.
Or if you don't have the necessary expansion, maybe just use a rental lot?
Downside of options 2 and 3 is that there will be walk-bys, which means a higher risk of exposure.
And general tip, use the group feature to direct the necessary sims to the right places (away from the rest).