The best options are either:
-Just basically lie and say "No I'm not seeing anyone else." For this to work effectively, you shouldn't really need too many alibis. Then continue to meet other girls and do what you like. For best effect, establish from the beginning taht you regularly have parties, or other girls coming over regularly so if she ever finds hairs in your bed or house that can be explained away. After that just empty your trash regularly. This is basically the "Japanese way" of treating the situation.
-If your Western guilt and morality is holding you back from this, then just go mostly honest. Tell girls "I might see other girls occasionally, but I'm interested in you and want to know you more." It's polite to not shove it in their face, of course.
In my case, I have literal large parties and streams of girls coming both into my house and to clubs and bars and events with me, so there's almost no chance they will believe I'm monogamous. I also tend to tell girls I sleep with for more than a couple times that I coach nanpa and pickup. It's never been an issue, but I don't really hide it for the most part. Although I do have two girlfriends, one is a porn actress and the other is a very sweet, petite, young office lady. The porn actress would not care if I sleep around, but the petite OL wants a romantic disney relationship so in her case I just tell her that I have these girls around just to fulfill the needs of both my coaching/business, the events and clubs attendance, etc. She says "I know you'll never be foolish enough to touch the 'tools of your trade'". Anyway, from each according to their jealousy to each according to their desires..