The question of whether you are overreacting is subjective and depends on individual boundaries and values within a relationship. People have different views on pornography and what they consider acceptable behavior in a committed relationship.
It is essential for couples to communicate openly and honestly about their feelings and boundaries. If you have expressed that watching pornography feels like cheating to you and it hurts you, your feelings and concerns should be taken seriously by your partner. It's not fair for your partner to dismiss your emotions as "insecurity" without acknowledging your perspective.
Every relationship is unique, and each partner must be willing to understand and respect the other's boundaries and feelings. If your boyfriend is unwilling to stop watching porn despite knowing how it affects you, it may indicate a deeper issue with communication and mutual respect in the relationship.
Consider having a calm and honest conversation with your boyfriend about your feelings and why pornography is an issue for you. If he continues to dismiss your concerns without trying to find a compromise, it may be worth evaluating whether this relationship aligns with your values and emotional needs.
Ultimately, you have the right to determine what is acceptable for you in a relationship, and if you find that your values and boundaries are not being respected, it may be worth considering whether it's a healthy relationship to continue. Take some time to reflect on your feelings and what you want from a partner and a relationship, and make the decision that feels right for you.