I'm rather new to the genre and the terms. I know what harem means classically, but I'm hesitant about speaking about it on here as I'm sure the aficionados will rip me apart for using it incorrectly. What I can say is that the MC will be involved with several of the characters simultaneously (not having sex with multiple characters at the same time, I just mean in sexual relationships with several at the same time). Some of them will know about each other's involvements.
As for MC being the only one having sex with the love interests... I don't want to give too much away. But I will give you some hints: Julian has been introduced for a reason (his involvement is not finalized, but I'm pondering several scenarios). Also, they are working on an experimental libido drug which may produce inconsistent results. You can imagine if there were hyper responders... That's about all I can say.
Just out of curiosity, would you prefer that the MC be the only one having sex with the love interests, or would you like to see others involved?