Flaming = queer, gay, homo. So it just evolved from there.

https://en.wikipedia.org/wiki/Flaming_(Internet)

(https://autoadmit.com/thread.php?thread_id=4038672&forum_id=2#36590561)

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intr. slang (orig. and chiefly Computing). To rant, argue, or harangue, esp. via an electronic medium (such as e-mail or postings to a newsgroup); to send an inflammatory, abusive, or (esp. in early use) inconsequential e-mail or posting, usually as a hasty response or in a rapid, angry exchange. Also trans.: to send (a person) such a message. Cf. flame n. and adj.

1981 CoEvolution Q. Spring 31/1 Flame, to speak incessantly and/or rabidly on some relatively uninteresting subject or with a patently ridiculous attitude.

1989 PC Mar. 181/1 You shoulda been there. People were really flaming.

1989 PC Mar. 181/3 Flame (verb) has two meanings that refer to a loss of control at the keyboard. The first definition is ‘to argue or complain vigorously with another by using a computer’... The second definition is ‘to communicate via computer rapidly and obsessively on subjects both significant and trivial’.

1994 Internet World July 31/1 He's been flamed publicly..and the other New York providers raise an eyebrow when asked about the system.

2000 Out Nov. 26/3 Not to flame, but the female model you had in your ‘The Great Escape’ layout is one of the harshest, most ill-proportioned women I have ever seen.

http://www.oed.com/view/Entry/71018

(https://autoadmit.com/thread.php?thread_id=4038672&forum_id=2#36591200)

[at MIT, orig. from the phrase flaming asshole]

1. vi. To post an email message intended to insult and provoke.

2. vi. To speak incessantly and/or rabidly on some relatively uninteresting subject or with a patently ridiculous attitude.

3. vt. Either of senses 1 or 2, directed with hostility at a particular person or people.

4. n. An instance of flaming. When a discussion degenerates into useless controversy, one might tell the participants “Now you're just flaming” or “Stop all that flamage!” to try to get them to cool down (so to speak).

The term may have been independently invented at several different places. It has been reported from MIT, Carleton College and RPI (among many other places) from as far back as 1969, and from the University of Virginia in the early 1960s.

It is possible that the hackish sense of ‘flame’ is much older than that. The poet Chaucer was also what passed for a wizard hacker in his time; he wrote a treatise on the astrolabe, the most advanced computing device of the day. In Chaucer's Troilus and Cressida, Cressida laments her inability to grasp the proof of a particular mathematical theorem; her uncle Pandarus then observes that it's called “the fleminge of wrecches.” This phrase seems to have been intended in context as “that which puts the wretches to flight” but was probably just as ambiguous in Middle English as “the flaming of wretches” would be today. One suspects that Chaucer would feel right at home on Usenet.

http://www.catb.org/jargon/html/F/flame.html

(https://autoadmit.com/thread.php?thread_id=4038672&forum_id=2#36591203)