Interesting, I've never heard of having a 7th house stellium can indicate that a person will not marry. Well, I have Sun, Mercury (both on Desc) and Mars in the 7th house. I entered my 40s and I'm not married - so there's that Your question relates to something I've been thinking about lately, but from a different perspective. I never thought I would not get married, because having crowded 7th house means that I am the happiest when I'm partnered. However, I started thinking that I might have to bypass the 7th house if I wish to find a suitable partner. I'll try to explain...
First off, I am sure it matters a lot what planets are involved. Perhaps having more "passive" (e.g. Moon) or generational planets means that the person's own power (for the lack of a better word) lies somewhere else in the horoscope. In that case, having a partner described by the 7th house wouldn't mean projecting / giving away your own power. Also, men and women probably feel differently about projecting their own qualities on their partner.
In my case, the most "powerful" point in my natal chart is on my descendant - Sun and Mecury in aspects with Uranus, Pluto and Neptune. And as I said earlier, my Mars is also in the 7th house. What happens to me if my partner embodies those energies? I would be overpowered, and the stellium belongs to me after all, not him. Btw, I am a heterosexual woman. Relationships matter to me, a LOT! And I had a good number of long term relationships. When I was younger, I thought my ideal partner would need to be the most charismatic, intelligent, energetic man on Earth And my exes were pretty cool, they fit the description of my 7th house - fiery and smart. I'll add that I am generally very selective and despite needing a relationship badly, I spent a lot of time single.
Long story short, I started questioning everything after my last relationship ended. We were very compatible, peers, best friends, there was so much joy... great school-book synastry, good composite. He was the perfect description of my 7th house. I won't go into details about what happened, but we are still friends and he recently told me - "We are both A-types, we should probably look for partners that are the opposite". He has a good point... Btw, he has Venus, Sun, Mars and Moon in his 7th! I loved him more than anybody, but being a woman, I felt like I'm giving away some of my power to him. I wanted a family, which would put me in a more vulnerable position, and I didn't like the idea of falling into his shadow because of that. So, my point is, it makes sense to me that a person with a 7th house stellium is in a pickle and might stay single in order to preserve her own identity.
I started experimenting with looking at that pickle from a different angle... and I am really curious about what others have to say! And I'm talking about natal charts where the 7th house stellium is a strong, action oriented part of the personality... is it better to look for a partner that is actually a little different, and more complementary? I read these articles recently, stating that it's not uncommon for married couples to have MC/IC connections.
http://theinnerwheel.com/2009/09/29/synastry-studies-the-m-c-i-c-part-one-m-c-i-c-m- c-i-c/
http://theinnerwheel.com/2009/10/13/synastry-studies-the-mcic-part-two-aspect s-and-inter-aspects/
Maybe focusing on MC/IC line can work better for someone with a 7th house stellium. I am currently really interested in a man who doesn't exactly fit my 7th house description but has a stellium on my MC. In an earth sign - which is the element that I completely lack. We shall see, I want to get married, and all those fiery A-types didn't really work out for me