The player has full control over all aspects, everything can be avoided if the player chooses and if avoided it does not happen in the story.
Yes lesbian and bi love interests can be avoided, they can also be added to the harem and do nothing with the other harem members if the player chooses. This novel will focus on the interactions between the main character and the love interests and not between the love interests themselves.
For example, in a sexual scene with two love interests and the player will be presented an option for them to interact with each other. If the player selects no, they will simply wait their turn for the main character with no kissing, touching or other interactions between them.
There will never be a sexual interaction happening within the story that does not include the main character. I hope this provided you with the details you were looking for!