Walked in on having sex - why was he angry?

Anonymous
I have been walked in on having sex. My sister walked in on me having sex with a guy I have been casually seeing for 5 years at the time
When we first met, he had some feelings towards me but in the year, but them feelings disappeared. He said I am nothing but a "side chick" to him and that we will only sleep with each other

So when my brother and sister in law were on holiday, i bought him into my family flat and had sex with him on my brother and sister in law's bed

That night my sister knew what I was upto so she barged open the door. The guy then pulled himself out of me then slapped me.. He seemed angry

When he was kicked out, i texted him a few days later and he threatened to come to my flat and beat me up

Why was he angry? Its not his flat nor is it his bed...

I was the only person in trouble as my entire family found out and I had to pay for a replacement mattress for them
Walked in on having sex - why was he angry?
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