A try from Nathan White and a penalty from Robert Evans had helped Maesteg to a 10-8 half-time lead, despite referee Greg Morgan brandishing yellow cards to Steve Tremlett and Paul Clapham for killing the ball.
Tries from Chris Bowridge and Rhodri Milson put the game beyond Carmarthen after the break, before sin-bins for Dean Ronan and David Dyer saw Maesteg reduced to 13 men for 10 minutes in the closing stages.
A Maesteg spokesperson said: “I’ve never seen a refereeing performance like that in 30 years. But despite that, we produced an amazing display full of guts and courage.”
Leaders Builth Wells fell to a shock 21-10 defeat against strugglers Penclawdd, and Mumbles came out on 18-16 winners in a thrilling game between the two bottom of the table rivals.
Aberystwyth continued their dominance at the top of Division Three West, running in six tries on their way to a 36-9 demolition of Tumble. Crymych were the other big scorers of the day, with a 33-0 win against Laugarne.
Aberavon Quins remain on top of Division Three South West after their 39-5 win against Briton Ferry. Second-placed Tondu are hot on their heels after scoring nine tries as they thrashed Seven Sisters 59-0.
Division Four West leaders Llandeilo are now red hot favourites for the title after a convincing 37-0 win over second-placed Tenby put them eight points clear at the top.
Bryncoch stay top of Division Four South West after they grabbed a 15-8 away win against Pontycymmer.
Cefneithin showed why they are top of the table with a massive 102-3 win over strugglers Swansea Uplands. The home side scored 16 tries in an emphatic display.
In the only game in Division Five South West, New Dock Stars beat Llandybic 24-22.
