Gene Simmons proposes to Shannon Tweed in Belize.

"Shannon, if we fire Tommy, will you play guitar in KISS?"

He's famously sworn that he would never get married ... but KISS frontman Gene Simmons is one step closer to breaking that vow ... proposing to his longtime GF Shannon Tweed in Belize recently.

We don't know what her official answer is -- but based on her reaction, we can take a guess.

The proposal -- caught on camera for “Gene Simmons Family Jewels” -- airs tomorrow night at 10 on A&E. Gene and Shannon met 28 years ago ... and have two kids together.

This according to the mighty TMZ

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