I have Venus in Scorpio square Pluto in Leo.  My relationships have always been very deep, but I hold back a lot of it.  I know I have too many deep and questioning feelings for most people.  I understand that many people might not want to go there.  Me neither.

Strawb, I think that a Pluto to your Sun is a very controlling energy, but even though Venus is receptive, they don't appreciate the invasion.  Most Pluto encounters are about power, in some way.  I think the Venus person will only give so much.  

The way you describe Pluto relationships as stick to your ribs stuff is so true.

Pluto, apparently - your or his Pluto aspecting the other's personal planets (personal experience).
Also, his or your Jupiter conjunct the other person's South Node by degree.
Generally speaking, the "hardest aspect bond in synastry to break" might as well be a synastry with A LOT of aspects.

I have crazy synastry with the ex. I would not say that we are soul mates but it was intense. His Pluto is conjunct my moon and ascendant. His Venus is at 15 Aquarius.  Mine is at 16. And I have Mars, Vertex, Jupiter and N. Node on his Capricorn sun. ETA: My Pluto is on his Jupiter and ascendant.  Squares his sun, sextiles his Sagittarius stellium.

Anything involving my Scorpio planets have been difficult to break since the bonds feel eternal and separating would "feel" like a death from either party. All of this hovers over my SN too so its extremely karmic.

The other's Scorpio Venus/Saturn/Pluto conjunct my Saturn/Moon/Pluto is at the top of the list.  "Til death do us part"  There is no doubt that there is real love (Venus/Saturn) but having it compressed by overwhelming Pluto and Saturn especially in such a possessive, obsessive, smothering and vindictive sign is beyond mortal limits. My Leo Sun is chained to the neck with this shit so I rebel and runaway quite frequently.  When times get tough, my Scorpio Venus man will stand by my side, no matter what.  He is always there to take me back and forgive me for my imperfect ways.  It is very difficult for me to part from such devotion and loyalty. The deep guilt that would consume me if I left him behind.

When I try to rebel against my Scorpio Venus man, I end up with much worse contenders:

Another person's Scorpio Neptune conjunct my Saturn/Moon.  It has been a maddening experience for my Moon to feel utterly nourished by Neptune's ocean of illusions. My Saturnian boundaries and sense of reality completely disappears. My Saturn is deeply attached and deeply obsessed with the ethereal connection. It's like time has stopped or time just doesn't exist. It takes me years to try to detach from this aspect but how can you detach from something that isn't real??  I've had experience with two different people with Scorpio Neptune and both had composite Venus conjunct Pluto/Uranus with me.. It was just absolutely ridiculous. Our connections were telepathic.  If I had a delicious thought in my head, the other person would get an erection instantly without me being present. We would dream of each other and confirm it in person the next day of the exact sequences. It was excruciatingly painful to let go of such an extraordinary and rare connection. 

I've experienced the dreaded  Venus opp Pluto and to this day when I think about that guy, I feel the yearning rearing its ugly head.  Fortunately for me he was such an asshole that I could break away, plus we didn't have great synastry in general.  However I am sort of interested in this guy (his planets in first column) and am wondering if the good can overcome the bad aspects:

AriesChick that's some serious synastry but how does his Mars aspect your Venus? Speaking from my own experience I would have nada to do with a man whose Mars doesn't slobber all over my Venus. I have learned the hard way.