Sorry, brother, she can be your mom but she is still a woman. Don't let her sexual needs put aside the fact that she has been there for you and loved you her whole life, as a minimum you own her respect.
About you personal fantasy, whatever is going on with you mom should not take you away from you sexual desire, in fact, you fulfilling it would be like karma but this time for your mom XD. However, dont make "her karma" your motivation and focus on your own sexual development.
About your mom thing and you feeling sick. Sorry but as the brother above said, while you are the one living under her roof, there is nothing you can order her to do to please you, but I want to try and give you a better option than just "suck it up".
1. I understand that this may not be very easy for you if you dont have the same mentallity like some of us here. You can see it as chance to deepen you relationship with you mom, your mom being sexually open with her partners in front of you can open a lot of doors. Maybe if you are somehow aroused by what she does you can also be part of that relationship in a cuckold way, just remember that this doesnt necesarilly involves "humilliation" or something like that, it can just be about being more sexually open, and making more beareable the whole relationship for you and your mom.
2. In case you are not interested on being part of that (which is completely understandable), you dont have to, however, you still have the obligation to respect her freedom. You must know one thing, biologically, one of the biggest fears of a woman is the judgement of others to their sexual nature, and this is even stronger when it comes to the way their offspring perceive her. Believe me when I tell you that, if that woman gets to know that you accept her whole being and not only as a mother but also as a woman, she will love and trust you as her son and friend like never before. You wont have to be part her her sexual life with her partner if you dont want to.
Now, I definitely recommend you to sit down with her and have a long conversation where you express all your inconformities or interests to her, but do it in a calmed and rational way, make it a CONVERSATION, neither a discussion or an ultimatum. Try to avoid anyway of making her feel judged, so she will be more open to what you ask her. For example, let clear that you respect her need and her relationship, but tell her that you dont feel confortable witnessing it while you eat, tell her you need space, and if possible if they can express their love in the privacy of her bedroom at least while you are in the house.
As a last advice, remember that she is your mom, she is possible the only being that will love you unconditionally in your whole life. Please do not let prejudice or some sexist societal expectations to destroy the beautiful and special bond between mother and son.
Do not hurt her neither as a mother and woman.
Sorry for the whole bible, I let myself go a little to much, but I hope being of help. XD
I thank any update to my advice.
