In a new interview with Rolling Stone, Billie discusses her forthcoming song, "Lunch" -- which details her being intimate with another woman. She says she first began writing this track before doing anything with another girl ... but was able to finish it after one eye-opening encounter with a female.

She says ... "I've been in love with girls for my whole life, but I just didn't understand -- until, last year, I realized I wanted my face in a vagina."

Despite opening up about her sexuality now, Billie's kinda peeved by the way her coming out made headlines in the press. Remember, Billie blasted Variety at the end of last year for outing her in their cover story ... even though she sorta facilitated it at the time.

BE adds ... "I was never planning on talking about my sexuality ever, in a million years. It’s really frustrating to me that it came up."

Now that pandora's box has been opened, however ... Billie seems far more comfortable talking about sex, including the topic of masturbation. In the interview, BE confesses to pleasuring herself regularly in front of a mirror, stating it helps with her self-esteem.

She explained ... "Partly because it’s hot, but it also makes me have such a raw, deep connection to myself and my body, and have a love for my body that I have not really ever had."

Billie finishes off this train of thought by hammering the point home ... saying, "People should be jerking it, man. I can’t stress it enough, as somebody with extreme body issues and dysmorphia that I’ve had my entire life."