I haven't done it yet this season because I'm not sure if it would be sinful or not
This can mean God is not making you satisfied to do it because He does not want you to do it.
But, also > what else is involved? Yes, there is the issue of if your gambling is out of greed, and you indicate you do it only for fun, not seriously in order to get rich. And it is wise to be satisfied with what we have >
"be content with such things as we have," we have in Hebrews 13:5.
Also >
"But those who desire to be rich fall into temptation and a snare, and into many foolish and harmful lusts which drown men in destruction and perdition. For the love of money is a root of all kinds of evil, for which some have strayed from the faith in their greediness, and pierced themselves through with many sorrows." (1 Timothy 6:9-10)
I notice here how it does not say
"the rich" get into such trouble, but
"those who desire to be rich" get in trouble. All you have to do is have the desire, and already . . . big mistake > possibly it is like how Jesus says that if we look the wrong way at a woman we have already done it. The evil spirit of just the desire will mess you up, no matter how much or little you have.
But possibly you do not desire to be rich, but still God is not approving what you do. God does personally guides us in His own peace, to do whatsoever He really desires to share with us. So, in case you are gambling but not seeking wealth, still it can be a way of having your attention away from personally submitting to God in His peace, and discovering all He desires to do with you >
"And let the peace of God rule in your hearts, to which also you were called in one body; and be thankful." (Colossians 3:15)
I will offer > submitting to God in His peace is much more interesting and challenging than betting
And sports betting could be part of a package deal of other activity which has your attention not where God wants.