If you found your sons have already been exposed to pornography, it is definitely the time to provide them with good solid Catholic education on sexuality. It would certainly be beneficial to have a father figure to assist with this education, but as a single mother, I think you also shouldn’t be deferring it completely to someone else either. Nowadays there are more and more good Catholic material for pre-teens and teens on this topic, many of them are available on youtube or video format. You can look up Theology of the Body (TOB) for Middle School as well as TOB for Teens to see if they are suitable for your sons. If your parish or a parish nearby has a good solid youth group, you may also want to consult with the youth coordinator there.