Under Family Code Sec. 4323(a)(1), cohabiting with a person of the opposite sex creates a rebuttable presumption that a the cohabiting spouse has a reduced need for spousal support. So, there is a risk.
Also, once your boyfriend is moved in, you would have to evict him with 30 days notice to remove him and the children (60 days, if living there for one year or more), and then an unlawful detainer action, which is a real civil action, not small claims, so it's expensive. And, there is no way to avoid this outcome.
Finally, under California law, cohabiting parters can sometimes be forced to pay support, if the complaining partner can prove a contract for something other than sex between the parties. And, if the boyfriend helps pay the mortgage or improvements in the property, then he would obtain an "equitable" interest in reimbursement for those contributions.
The only way to avoid the downside of the last paragraph above is to enter into a signed contract detailing the relationship rights and duties. Most people cannot manage to do this, because it's too emotionally charged. But, that's what you would have to do.
On the other hand, if you never leap, you never love. And, there are no guarantees in life.
Hope this helps.
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