I’ve considered this one a good test of mendacious jerk factor ever since I ran into a particularly loathsome creep on Usenet named Steve Fordyce, whose favorite hobby horse it was…

The marriage laws do not discriminate against homosexuals.
They have the same right to marry a person of the opposite
sex that heterosexuals do.

Now, everybody…including the bigots who make this argument by the way…know that this is a bogus argument. Let’s apply it to a different set of people…

Laws that prohibit the practice of Judaism do
not discriminate against Jews, since
Christians have to obey those laws too.

The problem is it sounds perfectly logical.  How can you argue that treating people the same is discrimination?  But it’s a fallacy of ambiguity. To say that you are treating everyone the same is not to say you are treating everyone equitably.  The trick here is that a word is being used in two different senses at the same time.  Look at this again…

The marriage laws do not discriminate against homosexuals.
They have the same right to marry a person of the opposite
sex that heterosexuals do.

The problem is with the word ‘discriminate’.  In this statement, it is being used in two difference senses at the same time.  Let’s look at its definition.  This one I took from The Free Dictionary…

discriminate

Verb

[-nating, -nated]

1. to make a distinction against or in favor of a particular person or group

2. to recognize or understand a difference: to discriminate between right and wrong [Latin discriminare to divide]

So in the one sense, yes, the law makes no distinction between gay and straight.  But it does not follow then, that the second sense of the word ‘discriminate’, to make a distinction against or in favor of a particular person or group is also not true.  Let’s rephrase it…

The marriage laws do not distinguish between homosexuals
and heterosexuals.  They give homosexuals the same
right to marry a person of the opposite sex they give
to heterosexuals.

This statement is both true and much clearer now as to adverse discrimination, in the first sense of the word, that homosexuals endure even though they are not being discriminated in the second sense of the word.  Let’s try it another way.

The marriage laws treat homosexuals and heterosexuals
equally.  Both groups have exactly the same right to marry
a person of the opposite sex.

Here the ambiguity is on the word ‘equally’.  Once again, it is being used in two difference senses at the same time…

Equally

adj.

1. Having the same quantity, measure, or value as another.

2. Mathematics Being the same or identical to in value.

3.

a. Having the same privileges, status, or rights: equal before the law.

b. Being the same for all members of a group: gave every player an equal chance to win.

4.

a. Having the requisite qualities, such as strength or ability, for a task or situation: "Elizabeth found herself quite equal to the scene" Jane Austen.

b. Adequate in extent, amount, or degree.

5. Impartial; just; equitable.

6. Tranquil; equable.

7. Showing or having no variance in proportion, structure, or appearance.

n.

One that is equal to another: These two models are equals in computing power.

tr.v. e·qualed or e·qualled, e·qual·ing or e·qual·ling, e·quals

1. To be equal to, especially in value.

2. To do, make, or produce something equal to: equaled the world record in the mile run.

 

‘Equally’ is being used to mean both Having the same privileges, status, or rights: equal before the law and Impartial; just; equitable.  But one does not necessarily follow from the other.  Let’s rephrase it…

The marriage laws treat homosexuals as if they were heterosexuals
and give them the same right to marry a person of the opposite
sex that they give to heterosexuals.

Now the problem is more clearly understood.  The marriage laws deny that gay people even exist.

The fallacy is one of equivocation.  It is using a word in two different senses, to prove a conclusion that does not follow from the stated premise, simply because the same word appears in both the premise and the conclusion.

A feather is light.
What is light, cannot be dark.

See how that works?  Now look at this…

Marriage laws do not discriminate between homosexuals and heterosexuals.
Therefore marriage laws do not discriminate against homosexuals.

It simply does not follow.  Yes, the law does not discriminate between gay and straight.  It does not follow that the law does not discriminate against gay people. 

Nobody makes this argument honestly.  Nobody.  This is bad faith on its face.  When you hear someone making this argument, you know you are dealing with either a bigot or an ass, and usually both.