In this situation, you would be discriminating against a single person right? If that is the case, they are not a protected class so you would be ok. At least, that is my opinion of the situation.
Not a violation, but probably a better solution would be to charge $35 per person as an Application Fee; then you would not have to be concerned about a fair housing violation. There are so many definitions of couples, married or not, that you could inadvertently set yourself up for a claim.
Here is a follow up to this post. What if they are getting married the week before their move-in date so your LC charges them the "married couple" fee?
Personally not a fan of this verbiage. Very "old school".
We recently had the same question come up in our company. We decided to eliminate the discounted application fee for married couples since it could be discriminatory toward a same-sex couple. Even though federal fair housing law doesn't recognize the LGBT community as a protected class, HUD does, so we decided to go ahead and make the change pre-emptively.