About 20 years ago, I had a very good friend that was LDS. Her daughter and my oldest daughter were best friends.
Also, while I was in college, my major professor (and advisor) was also LDS.
We both respected each other's religious beliefs and didn't try to convince the other person that they were wrong. My daughter and her friend (the LDS friend) often discussed religion, and the differences and similarities between them. They also went to each other's church activities.
That said, I don't know that much about the LDS church, but a lot of them are very moral people, and are very family and faith oriented.
Would it bother me if one of my family members married/converted? I'm not really sure... that hasn't happened to me. I would hope that they were making the decision after a true soul-searching..... and, since they were raised in a mainline Protestant church, I hope that would also come with a lot of prayer, rather than going on their feelings for that other person.
I think a lot of the discomfort comes from the aspect of their missionaries, and how much time they really try to convert people......
Just be aware... if your son starts asking questions of the missionaries, they will come to his house and start teaching him about their religion, with the BIG purpose of converting him. I remember a friend who, after finding out that her parents had converted, decided she wanted to "learn" more about the church... so the local missionaries came over and started presenting their lessons..... at one point, they basically said, "well, after 10 (?) more lessons, you can be baptized and join our church" .... that wasn't what she wanted, so she quit the "lessons".
I agree that lack of understanding breeds discomfort, and often dislike. Unfortunately, many people's view of the LDS faith comes from what they see in either the news, or on reality shows (the polygamists, or FDLS... which is totally different.)