Plenty of loving couples have threesomes... You're wrong to say it means they don't love each other even if they both agree to it. It's your choice of course, there's nothing wrong with not wanting to do it, but if your partner suggests it, you shouldn't assume it means they don't love you, unless they pressure you. In this case his boyfriend didn't talk to him about it first and clearly he was uncomfortable, so yeah, it doesn't seem like he loves him
It's a common fantasy. If you're worried about why they want to have a threesome, ask them why (and if they won't tell you, there's a problem in the relationship). The answer will most likely be something simple like "It's been a fantasy of mine for a long time, it seems hot and could be fun to do together if you're into it" or "I've had one before and really enjoyed it." If you've been together for a long time, there's a good chance the answer will be "let's spice things up."
Now this Is why I don't really like threesomes.. they must not actually like each other if they are willingly to have sex with another person with the excuse of love.. if my lover ever told me to have sex with someone I'd break up with them on the spot.. but I guess if that's what they're into.. sure