This is somewhat of a long story, but all the details need to be
spelled out for me to be confident that you have enough information to
answer my main questions at the end. I would prefer that you actually
know something about employment or contract law if you want to answer
it. Please list any credentials you might have that will prove this.

At my previous job, one of my coworkers secretly dated my boss for
over a year. During this time, my boss gave this coworker the highest
profile projects and seemed uninterested in my repeated requests for
career growth opportunities. After they announced their relationship,
my company separated my coworker from my boss organizationally and put
him/her under another manager within the same group. However, the
separation was only cosmetic with my coworker still performing the
same duties and regularly meeting with my boss to discuss project
specifics. 4-6 months after the announcement of their relationship,
they married.

Six months later, my boss got a new boss. A few months later, my boss'
boss made organizational changes and promoted my fellow coworker to be
my new manager. In my opinion, he made this decision due to the fact
that my fellow coworker worked on the highest profile projects which
were given to him/her by his/her spouse (boy/girlfriend at the time).
Since the manager that made this decision was not familiar with their
secret one year relationship, I made him aware of it soon after. He
did not seem to care.

My question is: Do I have a legal case against my former employer and
should I seek legal counsel?

In my opionion, my career prospects were damaged and my time wasted
due to the company failing to appropriately discipline my boss and
fellow coworker for breaking company rules and due to the company
failing to truly eliminate the appearance of a conflict of interest in
the workplace. I had an expectation that my company would ensure that
my colleagues would play by the rules or be appropriately disciplined
if they did not. Here are a couple of items that need to be noted: My
employer had company rules that stated that the appearance of
conflicts of interest in the company would be eliminated. Also, the
company's sexual harassment policy prohibited the behavior of my
coworker and boss.

I would also like to know if I would have a legal case even if these
rules were not in place. In addition, could this case be considered a
sort of reverse sexual harassment since I am of the opposite sex of my
coworker who married my boss? (I was penalized since there was no
possibility of me having a romantic interest in my boss.)

It's not sexual harassment unless one of the individuals in the
"affair" did not wish to participate.  They did remove the
"appearance" of a conflict of interest.  You can sue, but I doubt you
will get anywhere.

I know this is not what you want to hear, but you have absolutely no
case. Just because this situation seems unfair to you does not mean
that you have legal grounds for a suit.

To break down your claim,

You believe that you were not given the same opportunities that a
co-worker was.  You accept the promotion of your co-worker based on
their track record (or admit that it would be easy to make that case).

Basically, your only claim is that a co-worker had better
opportunities based on favoritism.

If your company has documented policies banning internal dating  you
might be able to make some sort of twisted case, but you would loose,
it would cost you money and you would tarnish your career.