The thing about a dating app is that it will ensure that you keep getting some sort of messages or updates or matches emails once in a while. Ensure that you set the tone for such a message as slightly different that your usual alerts. When you are sitting with your girlfriend on a date and you receive an update alert from the dating website, then just act as a little excited and interested. Check the update for a second or two and then remove the update. If she asks for what it is, then just act that it is nothing, just some random message. If your girlfriend is one of those who takes your phone and checks it, then make sure that you remove the update from the screen, she has no knowledge of the dating app in the first place, so she will return the phone feeling stupid. Being on a dating app gives you chance to actually get in contact with women who may be more interesting and less dominating or controlling that your current girlfriend. Decide yourself.