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It's quite sad, really, that in his 40's the boyfriend still cannot live his life openly for fear of his parent's reaction. He may feel like he can wait it out until his parents have gone to their reward, at which point he will start to live. Bad plan, of course. They may live to be 100, and he may die at 50. The real danger here, though, is that the boyfriend shares his parents' attitudes about the LW on some deeper level, and that there will always be resentment at not having an heir, etc. which will eventualy poison the relationship. So I think, in answer to the LW's question, she has a real basis to feel insecure.
I see no point in the LW threatening to tell the parents, much less carrying out the threat. It's the boyfriend's duty to do this, primarily to signify his commitment to the relationship.