Strawman

Got a really lame question that probably has a really simple answer: I know that if you multiply the concentration of Hydroxide ions (OH-) and Hydronium ions (H+) in any solution at 25 degrees it will always add up to 10-14. This is because they are 10-7 each. I've been told/read this is because if you add and acid to a solution (something that donates protons) they form Hydronium ions increasing the concentration of H30+ ions. Why does this decrease OH- concentration? HCl + H20 = H30+ +Cl-, no more. H+ increases OH- stays the same. Should be H+= 10-6 and OH- =10-7 = 10-13. I can't get to grips with why one affects the other. If it is because H+ reacts with H20 forming H30+, surely this has no affect on OH-. Is it a concentration thing? or is something else going on that i've not thought of? I need a really simple explanation please.