I volunteer in prison. When you are there, you put the crimes out of your head and focus on the human being in front of you. You see the unfairness in their lives - for example, most prisoners have some sort of SEN, and many are functionally illiterate. Most have trauma or abuse in their back ground. So you sympathise with this human being who had very few chances in life, and is suffering now. And you try to build them up, encourage them, plan for a better future for them, give them hopes.
Add to this situation, the manipulative skills many have. For some, the only skills they have ever developed. For some, they have relied on wheedling and coaxing trust out of people even to eat on the outside, from childhood up. For some, they find themselves for the first time, possibly the most attractive proposition in the room, when they are used to being the dregs of any social group.
Add to this the fact that some men genuinely change while they are in prison, and make good resolutions and stick to them for life. Some men make good resolutions and stick to them as hard as they can, but fail, fall into temptation, take drugs again, take money the easy way again, drink alcohol again. And some men never make good resolutions but pretend to do so, and I certainly can't tell which is which.
So you have a hopeful optimistic women, used to putting crime out of their mind, interacting with men who have had the worst start in life possible, and you rightly feel sorry for, some of whom will do well, some of whom are skilled manipulative liars, and you can't tell which is which...
Volunteers in prison need careful supervision, and we have to watch ourselves and each other for grooming. We debrief after every visit, and identifying possible grooming is part of that.
If you don't have that careful supervision, yes, I can see how these totally inappropriate relationships happen. It can be lethal both for the man and the woman. I was with a prisoner last night, for example, who's whole hopes and plans for the future are based on moving in with his "missus" and how she is going to help him get back on his feet and turn his life around, He is going to get qualifications and a job, and they will raise a family. I was some way into this conversation when it came out casually that these two people have not ever actually met in the flesh yet. They are both fantasising, and the outcome is likely to be devastating for both of them