Legal questions about a sign at a restaraunt

Example of such a sign. A couple questions:

1. Is such an offer legally binding? Given the fact that the other two offers are likely treated as an actual advertisement I am inclined to think so.
2. Is there anything legally preventing them from refusing to serve anyone who actually attempts to use the deal?
3. Could you just claim someone is your girlfriend for the night? That seems somewhat unfalsifiable. Could they refuse to honor the deal if they suspect the relationship is less than legitimate? I imagine this could open them up to a lawsuit if they have a pattern of preferring straight relationships.
4. Some older people use "girlfriend" to mean "friend who is a girl"
5. Legally speaking, would they be required to honor this in a gender non-specific way? Like a woman who brings a husband and a boyfriend? Or is it technically non-discriminatory because a woman could have a wife?