Is it safe sex if I ejaculate over the breasts of my lover? She claims it is not. I disagree.
It's pretty safe, as safer sex goes. If your partner has no open cuts, abrasions, or sores, for example, there's a negligible chance of any pathogen getting into her body. The odds of transmission of an STI are very low with this behavior.
Beyond the possibility of an STI, you may want to consider talking to your partner about her thoughts. What are her specific concerns? In what ways does she consider this to be unsafe? It may be that she is less concerned about safety and simply not be very interested in this particular activity. If that's the case, is this something that's necessary for your sexual pleasure? If her pleasure during this act is not equal to yours, could the two of you consider finding something different that you both enjoy?
If this sounds like a difficult conversation, try adding a degree of fun to this pleasure exploration. Each of you could make your own top 5 or top 10 list of new things you would like to try together. Then compare lists to see if you have some overlap. If that overlap doesn't exist, talk about why some of the activities are or aren't on your list. This could spark some new ideas and help you learn more about each other and your turn ons and turn offs.
Starting this conversation could open new doors for each of you and lead to a happy medium where both of you are giving and receiving pleasure.Alice!