If you can't define love, how can you say when it is not present?

Love can be defined to some degree by a strong level of emotional concern for another human being. It takes various forms. You can tell when it's not present when one person very clearly has no degree of concern or deep emotional bond with another person.

And "obvious" to who?

Certainly by anyone who is part of the relationship in question. Behavior is quite distinct when love is or is not present.

Where does your statement leave those who are incapable of having "strong emotional bonds" with others?

Why would you think that some people are incapable of this? All humans are capable of emotions, and love is an emotion.

Shall we just heave folks like that, into the landfill, and chalk them up to being not fully human or something?

Emotions are a part of human nature. But emotions will vary between different individuals. And no, I don't think those who do not love someone cannot experience a positive sexual encounter with them. Love does not have to be present in order for pleasure to occur. But when one person loves the other, and that other does not reciprocate the feeling, then heartbreak and even abuse can occur (but not always).

There are plenty of people who are incapable of having strong emotional bonds, with anyone at all for the matter. But they're people too. And if they feel love by having consensual sex (or any form of physical contact) from time to time (with even a complete stranger - who may also likely be the type incapable of having strong emotional bonds), then what's wrong with that? Are they less important than anyone else because of it? Do they deserve any less respect or rights? Etc.?

Why would you ever think that I would feel that people who only have a sexual interest in others do not deserve rights or should not be considered human? I challenge you to find any instance of me saying such a thing, ever. And I never meant to imply such a thing. I have simply said that there is a difference between a genuine GLer and someone who has nothing more than a raw sexual interest in girls, but do not particularly like them as people, do not find their social world or subculture endearing and appealing, and do not take an interest in all aspects of them outside of the sexual. Such an individual would be fine with a girl if all she wanted from him was a sexual experience, but if she was looking for something deeper, then I believe she would be better off seeking out an actual GLer, who was indeed capable of feeling more for her than just a raw physical attraction.