As mentioned above, I have this placement. So I'll start things off.Retrograde Mars in Gemini in the 7th House
(opposite Ascendant and Sun, square Midheaven)
This placement often makes me feel as though I experience all of the things Mars rules (sex-drive, aggression, desire to succeed,etc) on a much stronger level than the average person....but internalized. It's almost as if those things were meant to live within me and can go horribly wrong when attempts at expression are made.
I feel those with a direct Mars tend to feel the presence of Mars arrive when in a Martian-related situation, express that Martian energy, and then move on (until that energy is needed again). My retrograde Mars on the other hand seems to have a constant presence within. I feel I should be feared (despite rarely ever being aggressive) because I think Martian things all the time. I feel like an extremely sexual being (despite being a virgin with very little experience with intimacy) because the sex-drive of Mars mingles with my basic energy and rarely ever leaves for any reason. You get the idea. All of the energies that Mars governs are rarely expressed, so they build up and thus sort of lives within Retrograde Mars all the time. However, as unique as this all is it does not blend very well into a world where this is not the norm. A feeling of being ran-over or finishing last is common with a Retrograde Mars. It gets to a point where it's like, what good is feeling all of this energy if I suck at expressing it? Or at least expressing it in the commonly accepted / understood manner.
For example, I find that with this placement I have difficulty expressing anger / aggression during conflict. In hindsight I would always wish I had been more assertive (often in a witty / strong verbal sense since my Retrograde Mars is in Gemini). I imagine a Retrograde Mars in Aries would tend to wish they could have been more assertive (but by punching the other party in the face lol). Yet though Retrograde Mars has issues expressing aggression, one must remember the Martian energy is, in reality, very strong within (because it is rarely released, just built upon). This is why I worry that many of us have the potential (if not checked) to eventually kill or seriously harm people much easier than others based on repressed anger. Mars is our inner soldier, and Retrograde Mars is the quiet calculating soldier (reflecting Scorpio, the other side of Mars) while direct Mars is the brave aggressive solider we are all familiar with (Aries). So that's a little of what I've experienced with this placement so far. I would love to hear other opinions and experiences.