The /ð/ sound is spelled with <th>. The /m/ sound is spelled with <m>, <mm> or <lm>. Sometimes a final /Ɵ/ is voiced when made plural, which is why there are no matching singular forms for the pairs oaths/ohms and paths/palms. Notice how the homograph mow makes pairs with though and thou.
This is a contrast between a dental fricative and a bilabial nasal continuant, both voiced. It is not a problem.
Interesting pairs include:
The mean density value is 1%. The list makes 47 semantic distinctions, a loading of 63%.
booth boom booths booms bother bomber bothers bombers breathe bream dither dimmer dithers dimmers farthing farming father farmer fathers farmers further firmer furthest firmest gather gamma gathers gammas lathe lame lathes lames lather lama lathers lamas lithe lime loathe loam mother mummer mothers mummers northerly normally northern Norman oaths ohms paths palms seethe seam seethed seamed seethes seams seething seaming slither slimmer slithers slimmers southern summon teethe team teethed teamed teethes teams teething teaming teethe teem teethed teemed teethes teems teething teeming than man that Matt that mat thee me their mare theirs mares their mayor theirs mayors then men there mare there mayor they may thine mine this Miss though Mo though mow those mows thou mow thus muss thy my thyself myself tithe thyme tithe time tithes times with whim worthy wormy worthier wormier worthiest wormiest Worthing worming writhe rhyme writhed rhymed writhes rhymes writhing rhyming writhe rime
John Higgins, Shaftesbury, November 2010.