I do understand where you are coming from, I have an 18 year old son myself. But this situation sounds remarkably similar (almost identical) to what our family has been through in recent years with our youngest daughter.
Our daughter was in FE - about to move onto Uni. She had grown up with money whilst her choice of partner had grown up in care, lived on benefits, no intention of working, sole ambition was to get a council house and continue to live off either my daughters wages or benefits. Family background involved "broken relationships, half-sibs", children being removed and taken into long-term care, etc. When she first got pregnant -they both appeared genuinely 'devastated'. They broke up numerous times initially and got back together at his instigation. He was (at that time) very domesticated but not academic. My daughter was stunning, turned down a modelling career to stay at school and college, was very academic - straight A's all he way throguh school, etc. Whereas he is a scrawny little weed. She had her own money (earnings, savings & trust fund - all gone now). At that time we owned our own large house and had a considerable income.
My point is, our stories seem remarkably similar but whilst, at the time, they both insisted that they were using condoms, the pill, etc - five years later - they have admitted that it was all planned. Within four days of meeting each other, they were planning to have a baby, for her to leave college and go into full time work to support him.
It is possible that your son, especially if you have had a good relationship with him, is likely to be telling you what he thinks you want to hear and it may be some time before you hear the full story. All you can do is support him and even though our situation ended badly - (we have full, permanent residence of our grandson and he will never be returned to his parents care), if you carry on seing this in terms of 'fault' you must focus on both of them. They were both equally responsbile in that they both had sex. Even having lived through our experience, I would still say that I doubt very much, that she has 'trapped' him. These things happen and if you can't get past this part, you won't be able to give them the support that they are going to need without continuing to view her negatively.