More than one?

So, what is the real issue other people have if someone has more than one lover on the side? Does it affect any portion of their lives in any way? Are they jealous that they either have none, or that_ Armida:I found out because he post it on SC and I ask him about it.(2 December [...]
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Mana:
She's not very active but she's working on it.
(2 December 2018)

Mana:
There are some fights occasionally, some big but mostly small.
(2 December 2018)

Mana:
She's a beautiful girl inside and out also, but I think I have more in common with her roommate than her.
(2 December 2018)

Mana:
My girlfriend and I have been going out for a few years, everything has been fine.
(2 December 2018)

Mana:
She has a normal 9-5 job, unlike my current girlfriend who is a research scientist and spend a lot of her time in lab.
(2 December 2018)

Mana:
She has influence me in such a way that I can't explain how she did it.
(2 December 2018)

Mana:
She's also very active, swimming, jogging, etc.
(2 December 2018)

Mana:
She's a really sweet girl and totally down to earth.
(2 December 2018)

Mana:
She's the most beautiful girl I ever met inside and out.
(2 December 2018)

Mana:
And I think I have a crush on her.
(2 December 2018)