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I am certainly no expert, I am not even sure titfights are real or more erotic fantasy than anything else. But I would have to say that it really would depend on the fighters. Certainly lesbian or straight the women could become aroused by the titfight but that doesn't make them lesbian. After all for most women breasts are an erogenous zone. And Generally titfights are about which woman has the better breasts. Almost by definition it is a sexual battle. But that doesn't mean the women are attracted to each other just because they titfight. Almost they opposite for straight women I believe. At the same time, for Lesbians it seems reasonable that a titfight would be a fun little bit of foreplay for them. But in that case it really isn't a fight per-say. More like competitive intamacy. So I'd have to say that there is no straight/gay definition of a titfight. If the women are straight, then it is straight, if they are gay it is gay.
I agree with you that for some reason women are intimidated by large breasts and larger breasted women are instantly more competitive with other busty women. I personally think when Busty women hug they are usually competing a little testing their breasts against the other. Seems they hug just a little longer than women who are not competitive with their breasts. Both want to know they have the best breasts and that is their moment to know. I am sure both usually feel like they won the brief battle.