Much as it isnt pleasant to know that hubby has been exchanging texts with someone else, i believe it is an indication that he not completely happy with the situation at home, wants to discuss it but doesnt know how or perhaps isnt really fully aware of why he is not so happy as he once was.
By texting , its naughty but non risky it doesnt mean having to get involved physically with another person or be unfaithful.....................its a cry for communication between you, not a "get out of it" scenario.....
i truly believe that you should not take the moral high ground and end what appears to have been a good long lasting marriage........................but to acknowledge that perhaps things have slipped, adjustments need to be made on BOTH sides and use this experience to talk about how your lives can be enhanced together, going forward, a little more alert , a little more wary but perhaps also a littel more aware that all marriages can get a bit, well boring ???
Good luck