This is the ordinance that applies to the use of restrooms in the city of Houston. Note that to be considered unlawful, this has to be done "in a manner calculated to cause a disturbance".
CODE OF ORDINANCES CITY OF HOUSTON, TEXAS
Sec. 28-20. - Entering restrooms of opposite sex.
It shall be unlawful for any person to knowingly and intentionally enter any public restroom designated for the exclusive use of the sex opposite to such person's sex without the permission of the owner, tenant, manager, lessee or other person in charge of the premises, in a manner calculated to cause a disturbance.