No, it doesn't sound like you're being unreasonable.
If her bf stays 3-4 nights during the week and occasionally on weekends with his child, yours stays 1-2 nights a weekend, her bf is the one who's practically a 3rd roommate. Sounds almost like she's feeling like her weekends are being invaded. That's how I read it anyway. (Unless she just doesn't like your bf for some reason?)

Not to be a wet blanket, but guests who stay as frequently as 3-4 nights weekly might by some LLs be considered no longer guests and would be added to the lease. I'd probably add that person. Same for extended guests. I tell my tenants verbally at lease signing, when we're covering the clauses, that anyone staying longer than several weeks is not longer a guest but an occupant. And they have to be added to the lease. It's not about getting more rent $ either. (Actually, some of the reasons below impact you as well.)

One area is damage. (Also, the more regular occupants of a unit, the higher wear and tear over time.) So where it gets to be a problem, for instance, is say a BF is the type who has a bad temper. He gets angry and punches a hole in the wall or kicks in a door. LL is going to charge those on the lease for the repairs, because they are the ones legally occupying with a signed contract & who paid a Sec Dep. to be used for damage repair.

Another issue is liability. No doubt your lease contains a Hold Harmless clause, where occupants will not hold LL responsible for any accidents or injuries arising from use of the premises during their tenancy. But with someone who's there frequently but not on the lease, LL does not have that same protection.

Another issue is let's say the bf is a freeloader, consumes half your food, takes 45 minute showers, etc. He's racking up expenses that you & your roommate agreed to split. Only you can't afford to support the bf and carry the extra cost from his presence there. You want him to leave, your roommate refuses. You're reaching the point where you're just about broke so you discuss it with your LL. Some states and municipalities laws indicate that if a person has occupied a place for a certain number of consecutive days or weeks (it varies), then are legally considered a resident. And the only way to get them out is eviction. Even if they are not on the lease. So if freeloading part-time occupant bf refuses to leave, eviction is about the only recourse a LL has.

Just some things to think about.
It would be in your best interests to have her bf around a little less for reasons above. Hopefully you can come to some kind of compromise that serves everyone's best interests.