Romance in Persona 5 is easy to achieve. The protagonist can romance any of the female Confidants except for Sae Niijima. On the day that you rank up your Confidant level to nine with any of the romance options, you will get to choose whether or not to date them. This is very clearly denoted when your character says, "I should choose my response carefully." You can then either accept their love or shut them down.
Being in a relationship doesn’t change much. Aside from a special scene when you hit level 10, you can invite them on dates or up to your room to spend time together. Other than that, you will get a special scene with them on Valentine’s Day.
Choosing to date multiple girls at once will actually net you a bonus scene at the end of Persona 5. On Valentine’s Day, your girlfriend (or girlfriends) will invite you to spend time with them. Hanging out with one of them will net you a special SP chocolate.
If you are dating multiple girls, there will be a bonus scene on Feb. 15, where your ladies catch you cheating on them. They will beat the living daylights out of you, and Sojiro will give you a bonus chocolate to make you feel better. It is a funny scene but you will end up feeling like a jerk in real life. If that will weigh on your conscience, think twice before committing and have a drama-free Valentine’s Day with your special friend.