Landlord is trying to prevent bf and I from sleeping in one room overnight. We both have our own rooms. However I mostly just use it to change and store my stuff. He made a written agreement after I moved in and asked me to sign it.
The landlord cannot terminate the lease before the end of the current lease period (the month, on whatever day is the anniversary of starting the lease), since this was not a condition of the lease. You have been notified of this new term, so starting with the next month, it is part of the conditions constituting the lease, whether you sign or not. (However, you could explicitly "decline" this clause, i.e. overtly not agree to that term, then the landlord would decide whether to renew the lease without the objectionable clause). By continuing to reside there, having been notified of this condition you accept that term, so you are bound by that condition, which is not prohibited by Texas law. After the end of this month, the new term would apply, and eviction for violating the lease could proceed.