It's very clear that you respect your son's privacy. This is important and will provide a great foundation for continuing a relationship that is based on trust.
And you are right — it is a little tricky to strike a balance between respecting your son's privacy and bringing up the sexuality-related topics you overheard during his phone conversation. But it sounds as though you would like him to know that he can come to you to discuss anything — including sexuality. So, even though this may feel a bit daunting, you can find that balance!
Unplugging the phone probably wasn't the best solution — it could have sent a mixed-message to your son. He may interpret the fact that you disconnected the phone line to mean that you don't think he is mature enough to manage his time or make his own decisions. So it may be difficult for him to hear that you do want him to discuss very private things with you — and you would like to be someone he can go to for guidance.
You may want to let this one go, until you are both back on an even keel with one another. But if you feel that bringing the subject up will improve things, you may want to start by letting him know that you weren't purposefully eavesdropping on his conversation. Perhaps you can say something such as, "Last week I overheard some of what you said on the phone with your girlfriend. I wasn't trying to listen in, but I couldn't help hearing you talk about oral sex. I want you to feel comfortable talking about these issues with me, too." You could take this opportunity to talk with your son about how sex can be a wonderful thing that also comes with risks. There are physical risks such as sexually transmitted infection and unintended pregnancy. There are also emotional risks — some people feel vulnerable after they become sexually involved with another person.
Let your son know that you want to be someone he can come to with questions or to discuss his feelings. Tell him that you want to help him have as much information as possible. That way, when it's time, he will make the best decisions he can about sex — choices that will protect him and his partner both physically and emotionally. But you will have to realize that his decisions will be his, and they might not be the same as yours. Otherwise, the conversation will sound like another unplugging of his phone line.
Remember, many teens talk about sex with their peers. And parents can provide better, more accurate information. They can also offer a "safer," less judgmental opportunity for teens to ask questions without the pressure of embarrassment from people their own age. Beginning a conversation with your son now may set a good foundation for discussions in the future.
Your son is 13. He's starting his sexual life, and it's a good thing that he has a concerned parent like you. It would be wise to go to him and just talk, try and explain that it's healthy to express himself and if he needs anything to come to you. However, I agree with the expert, by blocking his phone time or unplugging it — you've just sent a mixed signal that may make him less likely to talk to you.
Be honest, open, and understanding, but also remember that your son is looking to you for guidance about the boundaries of what's appropriate and what isn't. He may want your input so he can explore what his own personal values are regarding sexual behavior and relationships. You can be the resource he goes to again and again, but only if he feels you're approachable and will not try to take control. Taking things away from him — like the phone — doesn't help foster communication in the long-term.