The "getting married to her" is a separate issue from tricking him into becoming a parent. Behaving like a saint until the "I Dos" is not an uncommon lament. Both men and women complain that once they get the blinders of love/lust off, they don't really like the person they thought they knew. If he suspects the oldest is not even his, it's obvious that hindsight is 20/20.
I applaud your husband for not going after his suspicions. Further proof that it takes more than sperm to be a dad.