There are many English words in which a final voiceless spirant (usually /f/) becomes voiced when the word is pluralized:
loaf > loaves
wolf > wolves
hoof > hooves
(This goes back to an Anglo-Saxon intervocalic voicing rule.)
What you're doing is applying the same rule to words ending in -th, changing the voiceless "th" sound [θ] to the voiced "th" sound [ð]. However, since these two sounds are spelled the same in English, most people aren't aware of this alternation, and the words that alternate in this way are not marked in dictionaries as irregular.
FWIW, I also pronounce "paths" as [pæðz]. Some other words that show this alternation, at least in my dialect:
cloth > clothes (though the plural "clothes" has acquired a specific lexical meaning, and the new regular plural "cloths" doesn't alternate in this way)
moth > moths [maðz]
